Yep!
As long as you have their permission, of course. But you said you've both been planning it, so it should be absolutely fine! I've done this quite a lot, actually. Most of my characters have relationships with one of my closest friend's characters, we make them around the same time but we already have established relationships with each other.
For example, two of our characters were dating and have been for several IC years - although we only made them during the last IC year. Hopefully that answers your question Lydia.
Looking forward to seeing you around, welcome to HS-Net!