Mains determine how the racket will play.
Poly has less power, feel and comfort. Poly has more spin, directional control.
Multi has more spring/pop , feel and comfort. Multi has less spin and directional control.
My personal guideline is if you are a baseline basher who only comes in to finish points when you have a big advantage in the point, then go with poly mains and the multi or syn gut in the cross.
If you are an all court player or have a slower swing speed or like to S&V, then use the multi or syn gut in the mains and the poly in the cross.
Also, some make the argument that no rec or intermediate level players should use poly. I don't agree with that approach. I think the vast majority on intermediate and above and learn to like a poly hybrid at low tensions especially if the multi is in the main.
There is not a correct way to do it. Federer uses gut/poly and Murray uses poly/gut.
Try a tension in the low-end of your racket's range and string the multi 3-5 lbs higher than the stiffer poly whichever way you go.