Since both have done doubles, let them show you the ropes. Of course chime in when you feel like it
FCs are good idea for doubles, however, if the ladies are not familiar with them, installation the first time can be tricky. If you don't use FCs make sure to have plenty of condoms, especially if the girls bring toys.  Are you a voyeur? It's fun to watch the ladies play with each other.  Four hours? wow I wish I had your coin. For me, I'd cut to the chase and just do an hour or 2 hour, but, that's me.  Enjoy!

Bring PLENTY of condoms.  When I do doubles with my bf  we can go through at least a dozen.  It seems half the fun for the guy is switching between us.

Especially for your first time with this.  And that you haven't had the two gals together.

What is important to you should be discussed BEFORE you play with them.  The reason is that many of the gals will do doubles, but they are NOT into each other.  But for the $$$$ will give you the illusion that they are DATY (and those that just aren't into it have "ways" to make it look real, but it is not!).  So I would certainly express my interests and desires to both ladies, and watch (carefully) how each responds.

OTOH, if they really are into each other, how will you be a participant in this game?  Perhaps you will be a voyeur, or a very active player.  Just let them know what YOU want, and remember this is all about YOU!

As far as how much time is good for this event, I would think that a two hour play window will be quite sufficient.  If they want to go to dinner...make sure it is OTC!  Otherwise, use those $$$ for another playtime vs dinner.  At least that is how I would approach it!

And if they are familiar with FC...absolutely the way to go