Troy is an insidious disease that causes all infected with it to vanish. Spread by touch, it runs rampant through the cities and countryside, and only ‘Sisters' with a resistance to its poison can endure its symptoms. Sister Elraine is one such young lady who is able to absorb Troy's poison for a short period of time, thus prolonging an infected's life, but like all Sisters she will eventually succumb to death at a young age. After beginning to have seizures that pre-empt this death Elraine is saved by a man named Kiri whose touch seems to negate the effects of Troy. Together, the two set forth to the Church as Kiri's immunity may be the key to curing the disease once and for all - but not if the organization Gazelle has anything to do with it..
A guy and a girl are forced to stay together all the time, or else on of them (or even both) will die. B-Shock is more mature in content, while Double Arts has more of shounen action adventure feel to it. Personally I prefer Double Arts over B-Shock, but if you find the theme I mentioned earlier interesting, you could give both a try.
A guy and girl are forced to stay together all the time or else they would be in danger of immediate death?
This happens to be the case in both B-Shock and Maga Tsuki. If you're interested in the theme you might want to give those two manga a try. Just keep in mind that B-Shock has some mature content at times and that Maga Tsuki is your (barely) above average harem series.