Deities are nothing but flavour-text. At the end of the day, they shouldn't dictate mechanical rules. There is no mechanical reason why I should be limited to one set of domains or feats just because of an arbitrary decision to include X and not Y for Z deity.
So why should I be restricted by such arbitrary associations?
If the restriction is in place for thematic reasons, then it isn't arbitrary.
If they allowed open selection of domains, they lose any thematic element to the deities at all. If they give deities too wide a selection, then the deity loses focus, and becomes "god of a whole mess of stuff". So, ideally, they would want deities with 2-4 domains (my guess - I haven't seen the book in question).
However, if you have some 20 domains in total, deities have a maximum of four of those, and characters choose two from the four, you need a lot of deities to make every combination a viable option. (Naturally, this is also at odds with the desire to have a compact set of deities for the setting, so as not to overwhelm players.)
The only thing that stops me, is that there is no Forgotten Realms deity with both domains.
What's unfortunate in this case is that the 4e Forgotten Realms has deliberately trimmed the pantheon right back to only a core set of deities. Had the full set been retained, there would almost certainly have been a deity to fit.
What I would suggest is putting this to your DM: my character would have followed
deity X (the pre-4e deity most likely to have those domains), but his portfolio has been taken over by
deity Y (the closest 4e deity). As such, would it be possible to follow that aspect of
deity Y, and so select these two domains?