The weight figure that decides this is the per-screen one

A dual arm can publish a per-monitor rating, a combined rating, or both. StarTech’s ARMDUAL2 manual gives both numbers: 17.6 lb (8 kg) per monitor and 35.2 lb (2 x 8 kg) for the pair, with the combined figure repeated on the product’s own warning label. Ergotron’s LX Dual spec sheet publishes a per-monitor band alongside a 40 lb (18.1 kg) combined maximum. VIVO lists 44 lb per screen for the STAND-V120T, WALI 22 lb per arm for the GSMP002N, and HUANUO 4.4–19.8 lb for each arm of the FlowLift.

So the order of operations is weigh first, shop second. Put each monitor on a scale with the VESA hardware you intend to hang on the plate, then compare the heavier screen against a per-screen figure — not the pair against a total.

These arms have a floor as well as a ceiling

Three of the seven publish a minimum as well as a maximum: Ergotron at 7 lb, HUANUO at 4.4 lb, and the Humanscale single-arm table at 6 lb. The other four — Jarvis, WALI, VIVO and StarTech — publish only an upper figure, so a very light screen has no stated lower bound to check against.

Where the two monitors differ in weight, the tension hardware is worth reading. Humanscale’s dealer documentation names Weight-Compensating Spring Technology, described as letting tension be matched to each monitor’s actual weight, plus a Counterbalance Indicator for setting that tension with no monitor attached, and Smart Stop joints that can be stopped along their rotation.

Screen size is a consequence of the weight rating

Ergotron states the relationship outright: typical LCD size is 27 in or under, the diagonal is an approximation, and the size may be exceeded as long as each monitor stays inside the per-monitor capacity. Ergotron alone publishes that caveat; for the other six the published diagonal is the only figure a buyer has, so treat it as the stated limit. Jarvis, HUANUO and WALI all publish 13–32 in; StarTech publishes a 32 in maximum; VIVO covers 32–49 in; the M8.1 crossbar is listed at 24 in in one authorized dealer’s spec table and up to 27 in in other retailer listings.

Panel depth is the specification nobody expects, and only Ergotron publishes one: monitor depth greater than 2.5 in (64 mm) may diminish the rated capacity, with buyers told to contact Ergotron directly in that case. No other pick here states a depth threshold, which means that for the other six you have a diagonal and a weight and nothing at all about the body of the panel behind the plate.

Which of these publish a thickness range

Three arms publish a thickness range. Ergotron documents three adjustable clamp positions — up to 0.63 in (16 mm), 0.5–1.63 in (11–41 mm) and 1.42–2.6 in (36–66 mm) — and includes a grommet mount kit (accessory 98-035) for through-hole mounting. StarTech’s C-clamp covers 0.39–3.46 in (10–88 mm), the widest published figure here. VIVO publishes both routes separately: C-clamp 0.4–1.7 in, grommet 0.4–1.5 in.

The other four give you nothing to measure against. HUANUO and WALI each list a C-clamp and grommet base without a range. Humanscale is described on a two-piece clamp mount base, and no thickness figure appeared in either authorized dealer’s otherwise detailed spec table.

If the number falls outside a published range, or there is no published range at all, the grommet route is the alternative: Ergotron, HUANUO, WALI, VIVO and StarTech all document grommet mounting, and of those only VIVO publishes a grommet thickness range.

Independent arms or one crossbar

Three picks are built as two separate arms on one base. Ergotron publishes two independent folding arms, each positioning its own monitor. Jarvis publishes two independently adjustable arms on a single base, each swiveling 360° and rotating between landscape and portrait. HUANUO publishes two independent arms with full-motion tilt, swivel and rotation.

VIVO’s STAND-V120T sits between the two approaches: two arms on a vertical center pole, each extending 17.5 in with 360° rotation and 180° swivel. WALI’s listing describes a dual arm with full-motion tilt, swivel and rotation, but does not state whether the two arms move independently of each other.

How much vertical travel is published, and how much isn’t

Jarvis is the only pick giving both ends of its range above the tabletop, but its separately stated travel figure does not match the span between those two ends, so one of the three numbers covers something else.

HUANUO, WALI and StarTech publish no travel figure in the documents retrieved. That is not a statement about how far they move; it means a buyer with a tall desk, or one who needs two mismatched screens to line up with each other, has nothing to check before ordering.

Two ultrawides are a different purchase

Only one pick here is specified for them. VIVO’s STAND-V120T is built for 32–49 in ultrawides with a minimum screen size as well as a maximum, 44 lb per screen, a 29.2 in arm span and tilt from +85° to -25°. That minimum is the catch: it is not a general-purpose arm you can also hang a 24 in secondary screen on.

The SKU is part of the specification

HUANUO sells a FlowLift Pro, also called the A5C, alongside the standard FlowLift, and the figures here are the standard FlowLift’s. The rest of the figures in this guide apply to the model numbers the sources name: StarTech’s ARMDUAL2, WALI’s GSMP002N, VIVO’s STAND-V120T, and the Ergotron LX Dual Side-by-Side listed on Amazon as ERG45245026.

Warranty length is published for four of the seven: 15 years for the M8.1, 10 years for both the LX Dual and the Jarvis, and 3 years for the STAND-V120T. HUANUO, WALI and StarTech publish no warranty figure in the documents retrieved.