The general limit on Citi applications is no more than 2 applications every 65 days. (There's a separate limit on Citi AA applicaions that's much longer, but it only applies to AA cards. Also it's "65" for safety; Citi claims 60, but they often make calendar math mistakes!)

You can (but don't need to) apply for two Citi cards on the same day. The possible (but not guaranteed) benefit of that is merging (on a per bureau basis) your two "hard pulls" of that day into one. But exactly how many "hard pulls" you have on your credit report is likely only to be of concern if you're applying for lots of other credit with other banks too.

Citi does not care if you have one, two, three, or even four of their Hilton HHonors cards open at a time. (They may not even care about five, but the highest reports I remember reading are of four.) So there's no problem in either:

• Applying for 2 of the HHonors 50k cards at once
• Applying for the HHonors 50k card and the Reserve card at once
• Applying for the second HHonors card a week or a month after for the first one
• Applying for two HHonors cards (at once, or a week apart, or a month apart), and then aplying for a third one 65 days later (65 days from the second-most recent application date)

Ie, anything that keeps you within "no more than 2 card apps in any 65 day period", with any mix of HHonors cards, should be ok.

(By "no problem"/"ok", of course, I mean if Citi "likes" you in all other aspects, they won't deny you simply because you have another HHonors card or simply because you apply for 2 cards at a time or 2 cards within 65 days. They will, however, definitely deny you if you apply for 3rd Citi card before 60/65 days.)