Yes, unfortunately it is extremely common and even many families right here on this board, while closeted about it, definitely expect to follow "one bedspread rule". Quite a few of the single girls have the expectation as well. Personally, even if the women claim to be bisexual, I would not be comfortable with sexual activity between sisterwives as it just totally complicates the relationships. That is to say, that type of relationship between wives, in addition to not being Biblical, would interfere with the friendship between the women and ultimately lead to divides in the marriages each woman had with the husband. I know both types of practicing poly families. The ones where the wives have no sexual contact are much more stable and healthy, living in the 'light' so to speak. The family I personally know locally the women do have sexual relations and there are dynamics created that have been extremely harmful in my view.
I have considered it at great length and I know for me, I just am not comfortable considering that type of relationship as it does not meet my definition of a plural marriage. Obviously, you know this about yourself too.
I think the best way to handle it is the way we have. Just be straightforward about the whole thing. Ask them from the beginning (together and separately I advise) what their expectations are in regards to sexual activity and sleeping arrangements. Generally, if they expect you to share all one bed, then they expect more than just sleeping in one bed even if they try to skirt around the issue. After they have satisfied you with their intentions, make it clear you are not bisexual and will not ever consider sexual activity with your sisterwife under any circumstances. A man who insists upon it and considers it a matter of obedience is, frankly, not entering PM for the right reasons and is looking only for sexual gratification and not the best interests of his wives. I believe with both sides being honest and taking your time to search for and listen for the clues, you will know what the intent is. Trust your gut instinct too... it is there for a reason and since I note on your profile you enjoy Mensa activities, I figure you are pretty well equipped to smell the poo when someone is shoveling it your direction.
Becca