Can 2 lights (same circuit), 2 switched, be converted into one switch?

I have a 3 switch plate on my wall. One switch is for an indoor light switch ( 3 wire, as there are 2 on the same line). The other 2 switches are two separate outdoor light fixtures. I would like to install an in-wall timer that controls both outdoor light sets at the same time. Is this possible?

Yes, there would be no problem doing that, if, as you say, the lights are powered by the same circuit. Be aware that many
electronic timer switches require the presence of the circuit neutral wire in the switch box.

casualfc said:

I have a 3 switch plate on my wall. One switch is for an indoor light switch ( 3 wire, as there are 2 on the same line). The other 2 switches are two separate outdoor light fixtures. I would like to install an in-wall timer that controls both outdoor light sets at the same time. Is this possible?

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I don't quite understand your current arrangement but I can tell you that the timer needs to go in series with the source for all the lights, and between the source and only these light circuits.
This can get pretty tricky.

Once you have a wiring diagram (not the same as a schematic) for your existing setup the necessary changes will be obvious.
Getting this diagram will be difficult, even if your wires follow the color code.
Make drawings of your existing setups in each box so that you can return to the original configuration if necessary, because the worst outcome is that you end up with nothing working and breakers that trip instantly.

You'll need clip leads, a DVM, an extension cord, and a test load, e.g., an incand. bulb. A helper will make things go faster.
I recommend getting one set of lights working at a time. Since switches are cheaper than timers, I'd substitute a switch for the timer until you're sure everything works as planned.

If you are describing what you have, and what you want to do, correctly, then all you need is the timer switch to replace either of the two switches, and then connect both lights to its "load" side, (if it directional and has one), and abandon the other switch and its "hot wire". Few timers require a neutral for the circuitry, since they use "backfeed" through the light for the neutral.

Thanks for the responses. Here's my plan in my layman's terms. In the wall are 2 switches, each attached to 2 black wires and a ground. The timer does not require a neutral, although it has a red wire for use in 3 way setups. My plan is to attach one black wire from each switch to one black wire each on the timer.
This leaves 2 questions. 1) For the unused red wire from the timer, can I just cut the wire, or cap it with a wirenut, or both? 2) For the 2 ground wires, can I just connect them and cap with a wire nut, or do I- connect the 2 ground and run a pigtail to the grounding screw on the timer, or connect the 2 grounds, and use a green wire connector with the hook from one ground going to the screw on the timer?
As you can easily see, this is new to me. Thanks for your help in advance.

casualfc said:

My plan is to attach one black wire from each switch to one black wire each on the timer.

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Post a link to your timer.

If your timer controls an internal DPST electromechanical relay then it should be easy. Otherwise you should confirm where each black wire from each wall box goes: to a 120 V source, to a neutral, through a load to a 120 V source or to a neutral, or to somewhere else. Measure with respect to a known good ground or neutral or hot lead while using an incand. lamp across your voltmeter leads.
Also check if there is 240 V between any two black wires, without using the the lamp.

If you hook it up anyway without knowing, because of the permutations and combinations possible with hooking up four wires, the odds are overwhelming that it will not work and/or you will clobber the timer.

At least wear eye protection.

It's this model timer.
http://www.intermatic.com/products/timers/consumer indoor timers/in-wall timers/st01c.aspx

I've also attached the installation portion of the instructions.

If a separate breaker controls each switch you can't use this timer.