How many splitters would I need for two subs?
You need a splitter for each channel, so two in your case. I linked AudioQuest splitters but if you check lower down the linked Amazon page, you will find no-name brands for about half the AQ version.
The splitter plugs into your rca output and then you run one rca cable per channel to the amplifier powering your main speakers and one rca cable to the low level input of each powered sub. This approach would give you stereo imaging for your subs, i.e., one sub for each channel, which is a common approach when running two subs. In that case I would position one sub somewhat near each main speaker.
Summing the sub signal is a typical approach used when running only one sub, and sometimes when running a swarm of subs (more that two) throughout the room.
