in Spades, when to bid such that the sum of bids equal 14?
There are no hard and fast rules or heuristics for such a decision, but the following indicators will tend to be positive in such a circumstance:


When one is clearly the strongest player in the game.
Strong players play well and weak players play poorly, of course; but under stress of an overbid deal weak players can be expected, typically, to exaggerate their own worst tendencies: this is part and parcel of being a weak player.
When behind.
In an underbid hand there are potentially 130 points available to be earned. in an overbid hand there will never be more than 120, likely less, points available. Anyone making their contract is thus automatically gaining on the field in an overbid deal.

In the most extreme case, where the opponents win the game if they make their bid and it is thus essential to set them, one might as well bid for the set and overbid the hand to 14.
When the hand demands it
If one's hand clearly is strong enough for a 3 bid with a chance at 4 tricks, but as dealer the hand is already bid to 11, a bid of 3 to overbid the deal at 14 seems in order. Someone is in for a nasty surprise  and it doesn't look to be you.
When long in Spades
Holding five or more Spades in your hand, on an overbid deal, will definitely surprise someone expecting length tricks in Spades that are now vulnerable to your holding.