It doe not really matter if the rates are the same or different.  Most of the ladies I do doubles with are in the same general rate range as me....some slightly less and some slightly more.  We all respect the other's choice for what they feel suits them, and no one ever feels slighted.  I personally prefer it, though if it is put is separate envelopes with each ladies name on them.  That way there is no need to count, separate it, or worry that someone will have hard feelings.  Things just seem to go a lot smoother.

Thus...my answer is that things just go smoother with separate envelopes and there is no risk that there will be any hard feelings.

...if it makes you feel better put it in separate envelopes.
I have doubles partners who are the same rate as me, some are 200 less and some 200-300 more.
Never hurts to double-check if the rate they post is for them to split or if you're paying their rates separately. I think it's case by case basis depending on who you're booking with.

Do doubles with on a regular basis, we have gentleman put our donations in seperate envelopes  
Our donations are usually about the same
I hav also met some of my favorite doubles partners through a client asking for a double with them  
I am open to connecting with new ladies but I always recommend my current doubles girlfriends first

My Doubles Partner And I Offer Different Doubles Packages From Our Solo's Packages  So That Is Never An Issue So We Usually Just Get 1 Envelope Then Split After.

my way to handle it
I tell the gent...you put her listed donation in an envelope with her name on it,....and you put my donation listed in another envelope with me name on it