Can the President summon a joint sitting after his notification under clause (1) at any time, regardless of whether both Houses are sitting?
Yes, the President can summon a joint sitting after his notification under clause (1) at any time, regardless of whether both Houses are sitting. This is mentioned in clause (3): "Where the President has under clause (1) notified his intention of summoning the Houses to meet in a joint sitting, neither House shall proceed further with the Bill, but the President may at any time after the date of his notification summon the Houses to meet in a joint sitting for the purpose specified in the notification and, if he does so, the Houses shall meet accordingly."