It is definitely a good idea to practice singing even while learning to play the Bansuri.
One advantage is that it gives you another way to internalise the positions of the notes (swaras), the form of the oscillations (gamakas), etc. It helps build a solid grounding when you have more than one method or tool to master the basics. The different forms of feedback help one to gain a better mental picture or model of the saptak.
In fact, the converse is also true: if one is learning vocal music, then it is often encouraged to learn an instrument side by side, for the same reasons.
Another advantage in your case is, as you mentioned, that it is another way to increase your lung capacity.
So, go right ahead and practice singing the saptak alongside your Bansuri practice! There won't be any dilution in your learning, only an enhancement.
One caveat: each person has a natural range for their singing voice, and it is possible that the pitch of the instrument is very far from the natural pitch of the singing voice. For instance, males typically have deeper voices and females more high pitched. So, it might damage the voice if a male sings at a high pitch for very long periods of time, or if a female sings at a very low pitch for long periods of time. Please take care of this aspect as well. In particular, if you feel a lot of strain in your voice by singing at an unnatural pitch, then please discontinue so that you don't damage your singing voice.
