Ethics for trip reimbursement from multiple parties
I would imagine that there are jurisdictions where getting reimbursed for costs that you have already been reimbursed for is illegal. It is at the very least unethical, so I think that thinking about this is certainly the right thing to do.

If you make a trip that includes more than the bare minimum "getting there and back" in travel (e.g., a personal detour on the way home, staying for the weekend after a conference for sightseeing, etc), many reimbursing institutions require you to hand in the receipts for your actual travel, plus estimates of what the bare minimum trip would have cost. For example, you could have printed out a screen shot of looking up a flight that just gets you there and back. They will then reimburse you for the smaller of the two amounts. 

Likewise, if you have a multi-stop trip, you can certainly ask to be reimbursed for an even smaller amount, as long as the total reimbursed amount does not exceed what you actually paid. How exactly you split the total is up to you, as long as no institution pays more than it would have if you had just traveled there and back. That's also ethical: if they invited you to a talk, they did so on the understanding that they will cover your travel cost to get there and back; if you are asking for less money (whichever those savings are split among all institutions), it's to their benefit, whether or not they get exactly half of the savings, or a different amount -- it's money they had budgeted for you, and are now getting back, nobody can complain about that.