This needs to be done in a 2-step process.
Rewire it so one of the switches powers both lights.
For this, we need to run a /2 cable between the switches. (or alternately, from the controlling switch direct to the light). If you use /3 cable, leave the full length intact on the useless wire, but cap it off. Could come in handy later.
Inside the non-controlling (temporarily defunct) switch box, take the cable (hot and neutral) that goes to the light (i.e. the switched hot on the old switch) -- and mark its black wire blue, and its white wire gray. On the new /2 cable, also mark both ends that same way. Note that we are marking the white wire gray.
In the non-controlling box, connect these two gray wires to each other, and these two blue wires to each other. They will effectively "pass through" this box without connecting to any switches. That will be permanently that way.
Back in the controlling box, you need to pigtail or legally double-tap the switch. Assuming there is one single bundle of white wires, the gray wire joins it. The blue wire goes to the switched side of the switch. (if it's on 24x7 you got the wrong side).
OK, so now you have both lights working from one of the switches.
Upgrade this to a 3-way circuit
Now, lay a /3 cable between the two switch boxes.
Then follow my instructions on the other post! Remember we will not touch the blue/gray splice in the far box.