You need all 4 runs from the tripple LNB structure, to a multiswitch.
Here is why... with the 3LNB (3 hockey pucks on the end), there are 4 Parts to the overall SAT signal. A,B,C,D
Each cable is only capable of carrying one at a time.
So what the multiswitch does is lock each one of the outputs to one of those parts.
Then the multiswitch intercepts the requests from the tuners (Be it one of the two inside of a Tivo, or one from a standalone machine), and sends it the appropriate piece.
Make sense?
(Note: I originally said 5 outputs, as you said you had two TiVos and where adding the HR10-250... I missed the part "upgrade").
So based on your setup of only having 2 TiVos.... If you want to use both tuners on both of them... You need all 4 from the dish... As TunerA and TunerB of the TiVo can ask for seperate parts.
If you only want to use one tuner on one of the units... then you can get away with only having 3