I was trying to look for the thread I saw a while back about this but I was wondering if there is a consensus
on handling the donation for a double if the providers have (slightly) different rates for that said duration.
I know two envelopes is a must and I figure I should just leave it at that.

Looks like I accidentally put this on the general board instead of the newbie board if a mod wants to move it accordingly that'd be fine.

You are making this way to hard. I don't really understand why this is a problem for you. 2 envelopes, each with the name of the woman on it it is for. No issues...

about setting up a doubles session, and let her organize it.

She'll let you know what the donation is, and then work it out with the gal she brings to the double.  That way you stay out of the middle.

On a few occasions, I have set up the double with each gal independently, and I paid them their published rate independently.

are OK with working together.  They may have a two for rate.  In this case, It would be OK to inquire so long as no discussion of activities, just their time.  If there's a doubles rate, you use one envolupe.  If they give separate rates, you package it according to their instructions.

Right my first doubles session went the way of the former which took that pressure on me I was thinking having flowers ready each with their name and their donation to the side to not leave confusion and to keep the donation unmarked as requested

right I never would be at this part if I didn't hop over that first hurdle. I posed them both the question individually and gave them each others information so they could at least introduce themselves to one another. I've already done it the one way before it was a lot easier (moreso because they were young co-eds at an apartment). I was actually thinking of something with flower(s) and keep the donation unmarked as requested.

with you will tell you the cost.  Put the $$ in one envelope and the ladies will deal with how to divide it up.