Shouldn't be a problem. You would just connect, say, both top wires from the existing switches to the one pole of the switch and the two bottom wires to the other pole of the switch.

The only trouble I foresee is some (many, all?) of those timers require a neutral to be present. Depending on how those lights are wired, you may not have a neutral in the box. You might also have a difficult time finding a wall plate to cover the missing device at your local home center. You'll probably need an electrical supply house or order one online.

It MAY be possible, depending on several factors. I would get an electrician to take a look at it.

If both switches are currently fed from the same incoming hot - pigtailed to both switches - it shouldn't be too tough. You simply wirenut the feeds to the lights together with a pigtail, and connect the pigtail to the one switch.

If they aren't wired that way - one or both on switchloops - then it gets quite a bit more complicated.

Others have pointed out the additional concerns here. I would add that some timers are powered by small internal batteries and thus don't need a neutral.