they are both at flirty mode but i dont see the option to "become bf/gf". When they talk the icon next to their green/pink bar is like two hearts linked together. What does that mean?
Pretty sure the linked hearts mean they are 'sweethearts'. It must be a bug/glitch that you have as I make sims and play them all the time and have never had any problem getting to bf/gf stage through to married BFF and soulmates.
When you made the sims in CAS did you check that they were just roommates to start off with or did you makes them spouse? If you made them as spouses then the game won't offer you the choice of them being gf/bf as they are already married!
If you look at the other sim in the relationship panel (the one for romantic relationships with the hearts- bottom right hand corner of the UI) you will be able to see what there 'status' is with each other.
Other than that, I'm really just not sure!