Two infant car seats on one stroller
For twins, or a newborn plus an older baby still in a carrier. Only a few strollers take two infant car seats, and the second position often needs its own adapter. Everything below is quoted from the makers.
UPPAbaby Vista V3, UPPAbaby Vista V2, UPPAbaby Vista (2015–2019)
Upper position on Vista 2015+ (not pre-2015): creates more space between the primary and second seat in double mode.
UPPAbaby Upper adapters for Vista (double mode) · UPPAbaby adapter page
Lower adapters for Vista 2015+ (not pre-2015); take Bassinet 2015+, Aria, Mesa V2 and Mesa Max (the page does not list the original Mesa or Mesa V3). Stroller folds with them on.
UPPAbaby Lower adapters for Vista (second seat) · UPPAbaby adapter page
Lower (second-seat) position on Vista only; not for pre-2015 Vista; explicitly NOT compatible with Maxi-Cosi Prezi.
UPPAbaby Lower Vista adapter for Maxi-Cosi, Nuna, Cybex and Clek seats · UPPAbaby adapter page
On Vista in double mode, a Chicco seat goes on top with Upper Adapters for Vista plus this adapter, or on the bottom with Lower Adapters for Vista plus this adapter.
UPPAbaby Chicco adapter for Vista and Cruz · UPPAbaby manual (PDF)
All car seats for the UPPAbaby Vista V3 →All car seats for the UPPAbaby Vista V2 →All car seats for the UPPAbaby Vista (2015–2019) →
Bugaboo Donkey 5
For the twin configuration with two bassinets; includes two sets.
Bugaboo Donkey car seat adapter (twin) · Bugaboo adapter page
For single and double (Mono/Duo) configurations with one bassinet; maker lists Donkey, Donkey 2, 3, 5 Mono/Duo and Donkey 6 Single/Double.
Bugaboo Donkey car seat adapter (single/double) · Bugaboo adapter page
Cybex Gazelle S
Two car seats on the Gazelle S (twin mode) need additional adapters.
Nuna DEMI next
Ring adapter mounts rear- or forward-facing in the upper mount; remove the infant carrier before folding. With the post adapters in the lower mount, close the canopy.
Nuna DEMI next car seat adapters (in the box) · Nuna manual (PDF)
Post adapters: close the stroller canopy when the infant carrier is in the lower mount.
Nuna DEMI next car seat adapters (in the box) · Nuna manual (PDF)
Mockingbird Single Stroller 2.0
UPPAbaby Mesa Max and Aria do not work in twin setups (two car seats); Mesa, Mesa V2 and Mesa V3 do.
Mockingbird Car seat adapter for UPPAbaby seats · Mockingbird help article
A seat that fits the top position may not fit the lower one. Check the adapter for each position before you buy.