Get a uncommitted diseqc and connect the 2 you have trough that. That is if you have a "advanced" enough tuner that can control a series like that.
I run a similar setup here with 2x4-ports committed and one uncommitted to select which way the signal should be sent (input 1 or input 2 on the uncommitted). You need at least Diseqc v1.1 controller on your sattuner for this to work..
Sounds complicated, but it's very easy!