Hey Luke, you'll get what you are chasing with the SplitMix, the Mark L’s Dual Stereo Line Mixer (is really not designed to do what you want) it will only give you 1 or 2 (sort of) stereo FX sends (using the duplicate Mix1 outs to Fx L/R and you then will need to connect short jacks from the other pair of Mix1 outs to mix 2 in to here the MP1 as you will have to return the Fx L/R outs to mix 2 (not Mix1 or it will feed back), and when you plug anything into mix 2 it breaks the internal connection from mix1 > mix2). Alternatively you can use the ExtIn (you must have MP1 in Mix1 - 1 input) to feed the Fx L/R and then you can return Fx outs to Mix1 - ch 2 or 3 or 4 as the ExtIn is a duplicate of the signal at Mix1 - ch 1 in only (so wont feedback)
If you want to go with a line mixer, the Alesis or similar will suit you better, 8 inputs, 1 x stereo Fx send
With the spilitmix you can add more Fx. All the SM inputs and outputs are stereo jacks so make some stereo jack to 2 x mono jacks (or buy insert leads). This is the way MikeB ran his rig for a while. The main reason he went back to chaining through his GMaj2 (4 CM style) was to pick up the GMaj2s CC (ie master volume control). You loose a little level with the SM coz it's passive but it's easily compensated for with the level controls on the MP1 and Fx unit.I posted a fair bit about this setup before (IIRC when you bought yours as well ?), my main goal was to keep the signal path from MP1/2 to power amp all analogue (as most more recent Fx units digitise the input).
How about the Alesis MultiMix 8?
FEATURES:
Eight-channel stereo line mixer
Microphone/line input on channel one
Stereo effects bus which can be shared between two linked units
Channel expansion feature for expansion to 16 stereo input channels
Individual Pan and Effects Send controls on each channel
Signal clip and overload LEDs on each input channel
Stereo five-stage LED ladder for output level indication
Stereo monitor out
Mono input source compatibility for stereo output
1RU 19-inch standard equipment rack size