A MODIFIED MONTREAL RELAY
The 3NT Opening Bid

3NT     Change happens... In the early days of bridge an opening bid of 3NT was a once-every-five years event.  (In those days we opened 4-card majors, too, but bridge has changed!)  But now, with the Montreal Relay, we open 2 and then bid 3NT, which means we can use the opening bid of 3NT for another purpose.  Hopefully for a type of hand that occurs frequently, too.

5-5 in the Minors
Let me propose that you talk to your partner and perhaps agree to try this:  With 5-5 in the minors open an Unusual Notrump call... 3NT.  You might have a hand like this:
If, instead, you open one of the minors, you may never have a chance to show the other one by the time the bidding gets back to you.  The opponent's will have found their major suit contract.

Yes, I know it has only 10 points, but the opening bid of 3NT is very descriptive for your partner and it has great preemptive power since the opponents have to start at the 4-level to find their major suit fit.  I would not recommend any point-count less than 10 points, and if that's all you have I suggest they all be in your two suits, as in this example.

You do not have to limit the bid in point-count, either.  Two-suited hands that are very strong are often difficult to describe, but this 3NT suggestion can handle strong minor suit hands, too.
Open 3NT and after partner chooses his best minor you can push it to a slam, knowing you are in the "Correct" minor.

Gambling?
And for those of you who prefer to use the bid as a Gambling 3NT, showing a long and solid minor, let me note that your bid occurs much less frequently than a hand with 10+ points and 5-5 in the minors.  (And the Gambling 3NT contract is always played from the wrong side of the table.)


Best Defense
This is not an entirely one-sided discussion... Below is the best defensive bids I know to use against players who adopt the 3NT bid as above:
  • Double is a penalty double for at least one of the minors if not both.
  • Cue bidding a minor suit is a major suit takeout call for either major, but cue bidding clubs, the lower of the minors, shows better hearts.  (The lower of the majors.)  Obviously then, cue bidding diamonds, the higher minor, shows better spades, but both cuebids ask partner to choose his best major.
  • Bidding 4 or 4 is natural.  (I have no idea what 4NT would be.)