If I were you I would leave them to it and step back from any involvement other than friendly. There should be no requirement to fall into their sexual energy and besides, it seems to work better to take things really slowly.
If this is hard for you, you haven't even met them and you are confused as to what you are even embarking on, then why rush? Let them get to know each other in the context of you being around with them, get to know her and get more settled into the dynamic before adding you being sexual with them. You might find you are creating more pressure than need be and that really you don't want that kind of involvement along the line.
Do a tag search here for "jealousy" "swinging" and anything else that looks interesting.
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