As Sal will be the only tummy mummy, we have been very lucky in that all our kids will be biologically 'the same' ( not that I truly believe it matters, but it is nice for a familly to look similar) We went through Melbourne IVF and have friends of ours that are about to start IUI with the same donor as their little girl, but with a different tummy mummy. They have incured more costs to do it that way, as Melb IVF insist on treating the second tummy mummy as a brand new patient, not as the partner of someone who has already done the journey, but they were quite happy to 'hold' enough sperm for their family to have as many kids as they want, regardless as to who is carrying them.
Good luck on your journey ladies, hope to see you in the preg announcements forum soon