Well someone is inevitably going to do it so why not start discussing it now? Ignoring differences in power levels for the moment, postulate possible plot hooks where a squad of Guardsmen and Deathwatch Space Marines would work together for the length of a campaign...
Knowledge is power, guard it well : Squad "The Party" is part of a regiment seconded to the Jericho Reach campaign. After a failed attack on a Tyranid world, the regiment is forced to retreat and effectively reduced to just "The Party." The fight to safety is brutal and forces the squad against several new strains of Tyranid but victory favors the Imperium that day.
Upon making it back... well the good news is that the squad is safe from any IG policking. The bad news is that this protection comes from Deathwatch/Ordos Xenos and comes at the cost of Deathwatch wanting them to do it AGAIN. Good news again is that a Deathwatch marine has been attached to the squad as an observer...