I guess it's a thing you could do, but there are good reasons not to:
1. EDA usually prohibits it (component collision). (Assuming these are separate components.) So you either ignore the warnings, or add rules to exempt those parts specifically?
2. Chip components butted together, will probably form a large solder fillet between them, potentially increasing stress. Especially if mismatched types, I suppose? Or kinda not, like: a resistor and capacitor together, the resistor doesn't have side metallization so at worst it makes a thin fillet around the edge of its electrode? Maybe this would only apply to capacitors and ferrite beads/inductors (having wraparound end cap metallization)?
3. Makes some ballsy assumptions about the accuracy of the pick-and-place machine. I suppose you'd at least nudge the centers a few 5 mils apart, so the one is unlikely to crush the other when it comes in. And they probably stick back together due to surface tension when soldering. Not sure if that's also a tombstoning risk? Seems like they'd mostly tend to do things together, it'd be hard for just one to go tits-up I'd think.
4. Likewise, it's a lot harder to service. You're probably melting both off, picking them apart with tweezers, and replacing new parts. Which inevitably glom back together again, of course.
Electrically, there's nothing wrong with that. If they're identical components, it's like it's a single double-wide component. They act in parallel, ESL is lower, etc. Likewise there's very little inductance between them, which might be of value say for very compact filters (possibly useful at GHz?). Power rating isn't quite double because they heat each other; there's a missing side for heat dissipation plus they're in each others' heat islands (i.e. where the heat is spreading out over the board / traces).
Tim