I agree with Quartet, especially the jet packs and the keys hint, although Hyperdine Side Arms is pretty close too.
From MAME info:
Quartet (c) 1986 Sega.
A platform shoot-em-up hybrid from Sega for one to four players, selected from either Joe (yellow), Mary (red), Lee (blue) and Edgar (green). The action takes place over a number of sideways scrolling levels, the bulk of which consist of straightforward platform action. On some levels, however, there are jet packs for the players to collect, and the gameplay's emphasis switched to that of a standard, sideways scrolling shoot-em-up.
The object of the game is to destroy an army of robots that have taken over an Earth colony satellite station. Each level has a boss character that must be destroyed. Upon its death it releases a door key that must be used to exit the level. Weapon and character power-ups can be collected to aid players in their task.
Due to its potential for four-player shoot-em-up gameplay, Quartet's gameplay feels vaguely similar to (though by no means as accomplished as) Atari's 1985 classic, "Gauntlet".
Side Arms - Hyper Dyne (c) 1986 Capcom.
An evil alien race called the 'Bozon' are trying to wipe out Mankind, and Lieutenant Henry and Sergent Sanders are given the responsibility of saving the Human Race. They each climb into a Mobilsuit - a jet-powered, heavily armoured combat suit - and set out on their mission.
Side Arms is a horizontally - and occasionally vertically - scrolling shoot-em-up for 1 or 2 players. Each player has 2 fire buttons, one for shooting ahead and one for shooting behind; crucial for a game in which enemies attack from all directions. Shooting certain enemies will reveal power-ups - displayed as 'POW' icons - and shooting the icons will either change their type, or reveal a special weapon.
Finally, Side Arms features a unique power-up. These power-ups are hidden in the level and can be collected by shooting at their location with any weapon. By collecting a circle icon that flashed between the Greek 'Beta' and ok, both players can join together to become a single, highly powerful combat unit capable of firing in 8 different directions. When a 2 player unit is joined, BOTH players' controllers can control the unit's movement, which can cause the unit to not move if players decide to go in different directions to each other. The player who picks up this power-up will control the main weapon from his current list only and the other player will be able to fire the secondary 8 direction weapon. The 8 direction weapon can be fired by single presses or automatically by holding either fire button.
In a 2 player game, this form will break apart after one hit to the unit, returning to both players individual units. In a 1 player game it will break apart after 2 hits, causing the second unit to be destroyed.
Apperance of the super unit and the 8 directional weapon is dependant on which player gets the power-up. First player unit is a primarly orange unit that fires red triangle projectiles one way. Second player unit is blue that fires blue boomerangs that fly out and return causing a slower rate of fire.
The 8 available power-ups are :
* Speed-up (POW symbol)
* Speed-down (Reverse POW symbol, shows just before reaching Auto I or II)
* Bit - (3 level power-up)
* Spread Gun - (3 level power-up)
* Mega Bazooka Launcher (2 level power-up)
* 3-Way Shot (2 level power-up)
* Auto I (Chain gun)
* Auto II (3-Way Auto)
Failure isn't about falling down, failure is staying down.