This is not a date, we would go as friends.
I'm sure she won't reject my invitation (as we been to concert before).
She does have a boyfriend, so she might think the tickets are for the two of them.
since you've edited to add this information, I think it is fair to update my answer to take this into consideration.
My suggestion is to give her one ticket and tell her you have one for yourself, if she would like to go together. She might have already start to plan to go with her boyfriend, this would give her a good way to both go with him and you.
This still assumes that the tickets are not linked to a specific seat. See here and following for a discussion on why this could be a problem.
In chat you say
Her having a boyfriends shouldn't matter, except that i might offer a date for them as a gift
I would try to clarify with yourself why you think that would matter, as Tom says in his answer for a similar reason, that might be a warning sign, also because later you add
The tickets, having an actual present (containing the tickets). So she know she is going when she gets the ticket. Not sure how I can ask her who else would be invited (since i already bought the tickets). Nor is this my issue XD.
I think this is very much your issue, though, why are you opposed to letting her go with her boyfriend? Or why would you not try possibly to make her even happier than you intend by having also her boyfriend with you two?
You also say
It is a seated concert, so I would buy two allocated seats.
that basically negates my assuption. Personally I think it is more appropriate if, as other mentioned, you would check with her beforehand, or at least with her boyfriend. If you do not have his contact, I'm sure you'll be able to recover it.
If this is not possible, then in my experience is better to let her decide if she wants to go with you or with her boyfriend, by giving her both tickets. You could still mention you would like to go with her, eventually saying something "as with concert such and such, I would like to go with you".