If you're not going to use Magic, I'd advise +Spd/-Mag or +Str/-Mag.
Double attacking is more valuable than extra damage, even in double duels. Though it would depend on who you're facing and their speed stats obviously if you even manage to double them or not.
In the case that they're fast, the extra Str would do you good for a few more damage, which is still better than standard. If you add Str to an effective weapon, you'll have an even better time.
I know what I'm saying is obvious, but really you just have to decide. It's a risk no matter what you pick for online battles, since the teams can vary so much in FE:F.