Since nobody was willing to give you a response on this tricky topic. Let me know give you my take.
I have defined these in two ways. Common ground and Uncommon ground
Common ground: Both denominations accept that the Lord as the Saviour and Christ
Uncommon ground: Difference in doctrine, beliefs, how faith is exhibited, role of women, gifts, tongues, healing and so forth
If you believe that the common ground is enough for a happy married life, then yes. there 'ideally' should not be any problem. But we do not live in an ideal world. The world is not perfect but with sin therefore what I have listed in the uncommon grounds part, would be an obstacle.
It is not that there aren’t any Pentecostal and brethren marriages today. But, if one who is in the 'brethren' community is convinced of the fallacies (not a topic of debate in this discussion) in the Pentecostal teachings. He/She would not usually get into an alliance. But then as stressed, we do not live in an ideal world and may the Lord provide one with the guidance.