Shower heads restrict flow already. The volume of water behind the head is significantly higher than the volume going thru the head, enough so that it can usually support 2 low-flow heads at once.

To see how this works, try timing how long it takes to fill a bucket underneath the faucet in the bath, then how much longer it takes to fill the same bucket with your shower head.

If it takes less than twice as long to fill the bucket with the shower head, then you won't have sufficient pressure to dual nozzle your shower.

This also means you are probably not already using low-flow heads, and it is likely that you are spending more on water than you need, and sending a lot of water down the drain for no reason.