If I read that right, you have one sub and one amp in the car and one sub and one amp in the garage. I will assume that the subs are the same brand and size. The same will be assumed for the amps. If that is the case:
1- I would add the second sub to the trunk with the amp already there. This is a lot less wiring than trying to do everything else. If the amp is running one sub now, is it bridged already? If it is already bridged check to see what the Ohm load is on your speakers and what Ohms the subs are. If it is a 4 Ohm sub and the amp won't handle 2 Ohms you will have to hook the subs up in series. If your amp will handle 2 Ohms and they are 4 Ohm speakers you can hook them up in parallel. It is worth it to do your homework. There are a lot of wiring diagrams online on how to wire up amps and subs. Google can be your best friend.
2- If you are not satisfied with the above, hook up the second amp. Put one amp on each sub. You can get away with a Y-splitter on the RCA inputs to the amps. The remote turn on lead can be split as well. The Power and Ground should go through separate fuses. You can do that the easiest with a small distribution block. Most amps now have built in fuses.
3- If the amps are not the same. If the larger amp is installed now, add the second sub. If the smaller amp is installed, try it first. Then switch out the amps if you want more power. Watch the Ohm loads.
4- Enjoy the system.
