You're planning to double whatever suit the opponents run out to. If partner has the strength to accept a limit raise, he'll jump to 4H or cuebid. If he bids only 3H, you'll pass. If partner has spade support, he'll bid 3S, 4S or cuebid. Over his 3H, you'll bid 3NT and let him decide.
Partner can pass with a minimum or bid game with extra values or good spade support.
The immediate splinter bid, though, is more descriptive and will help partner better evaluate his hand for a possible slam. The Unusual vs. Unusual convention is popular because it offers an accurate, flexible way for responder to describe his hand after an unusual 2NT overcall by an opponent. Although that use is the origin of the convention's name, Unusual vs. Unusual can also be valuable over other two-suited overcalls. The same approach can be used when an opponent makes a Michaels cuebid showing two known suits. Length in our lower-ranking suit.