These two are very compatible. They have many goals that are in common. Both of them look for life that is explorative. Both love to play with ideas, to get out, to be independent, to see life as an adventure, and to not be tied down. So, they have a lot in common from the very beginning.
Sex is very innovative because they both enjoy it and can think of wonderful things to do in bed. Many times it's not even in bed and they have that fascinating physicality, particularly Sagittarius, which is considered the sign of physical energy. Aquarius is the sign of mental energy. But their energies work wonderfully together.
They don't try to tie each other down, and that's the glue in a way. It's odd to say it's glue, but it's the center of their relationship, which is that they're not possessive. They tend not to try to dominate the other. So, this more or less care-free - of course, no life is care-free, but it's an approach to life where it should be bubbly and joyous and it should be something that gives happiness. Both of them approach life that way.
So, the care-free attitude, the sense of life as an exploration is something that keeps them together, not only sexually but as a friendship, as a business partnership, and as two people who can move through life with the same goals. Which is to approach life or to, let's say, experience and partake of life as a banquet.