For a bicycle:
* Bisikleta ng dalawa (literally "bicycle of two")
* Tandem na bisikleta (using the English word "tandem" as an adjective)
For other contexts:
* Sabay-sabay (means "together" or "simultaneously")
* Magkasama (means "together")
* Magkapares (means "paired" or "matched")
The best Tagalog term would depend on the specific situation and the meaning you want to convey.
For example, if you're talking about a tandem bike, "Bisikleta ng dalawa" or "Tandem na bisikleta" would be the most appropriate. But if you're talking about two people working together on a project, "Sabay-sabay" or "Magkasama" would be better choices.