Hard for me to do a diagram. I'm already done and it worked. I'll try to describe it again and if that doesn't work I'll try to draw it later.
Original wiring was two simple switches, each on had a hot wire (line) and a white wire (load). The load wires go to the fixtures. The neutral wires are in the box but are not connected directly to the switches.
New wiring- I left the white wires where they were on the old switches, but removed the hot wires from the old switches and attached them instead to the new switch. Then I took a new piece of wire and connected one end of it to the second terminal on the new switch and the other end of it to the empty side of each old switch. It works as I described- each old switch controls the one fixture just as it did before, but only if the new switch is in the ON position. If the new switch is OFF then neither light can be turned on by the old switches.
This last paragraph is not really relevant to the set up in general, so feel free to ignore it.
There is one other thing that I didn't mention... the new switch is a timer and needs a current at all times or the screen blanks out. In this set up, if both old switches are OFF then the timer can't draw a current. (In a normal set up, the timer doesn't have another switch behind it, so it is able to draw a current even while the lights it controls are off by relying on its high resistance). To work around this, I dug out a neutral wire from the box and connected it to another terminal on the timer switch.