Well why don't you come out to your friends and see what they say? Not come on to them, just come out to them.
The chances of you, as a couple, establishing a romantic sexual relationship with both members of another couple are slim to none. Heck, the chances of you, as a couple, establishing a romantic sexual relationship with one person "to share" are also slim to none.
Bringing another person into "our bed" is a common but misguided impulse in poly noobs. Most experienced poly people, coupled or not, date and fuck separately.
Probably not what you wanted to hear, but it's the truth.
Love withers under constraint; its very essence is liberty. It is compatible neither with envy, jealousy or fear. It is there most pure, perfect and unlimited when its votaries live in confidence, equality and unreserve. -- Shelley
me: Mags, 59, living with:
miss pixi, 37