This would surely be a rather desperate way to start a second rage within one encounter, but you could – after your regular rage has ended – draw a sword (or other weapon), cursed berserking (DMG, p. 276)
This item appears to have the characteristics of a +2 greatsword.
However, whenever the sword is used in battle, its wielder goes
berserk (gaining all the benefits and drawbacks of the barbarian’s
rage ability). He attacks the nearest creature and continues to fight
until unconscious or dead or until no living thing remains within 30
feet. Although many see this sword as a cursed object, others see it
as a boon.
It may be controversial, whether „going berserk“ counts as „a rage“ with regard to feats and other abilities but I think, the wording „gaining all the benefits and drawbacks of the barbarian’s rage ability“ suggests that it does.
I have to admit that using such a sword would be a bit, err …, hazardous. Your allies should keep distance and have some good strategy to get you out of trouble when things get totally out of control (cast Benign Transposition, maybe). But I could think of situations when drawing such a sword might be worth considering. Also, if you have Quick Draw, you don't have to spend an action before going berserk with that weapon.
It might also be an option to craft/obtain a cursed magic item based on the spell Mindless Rage (Spell Compendium, p. 142). This spell forces a creature to attack the caster. Also, ...
While under the effect of a mindless rage spell, the subject can make use of all its normal melee combat skills, abilities, and feats—either offensive or defensive. (...)
An interesting side effect of mindless rage occurs when the spell
affects any character or creature that has the rage ability (such as a
barbarian). In these cases, the mindless rage spell automatically
activates the creature's rage ability (and counts as one of the
creature's uses of rage for that day).
A magical item can be cursed in a way that it has an Opposite Effect or Target.
These cursed items malfunction, so that either they do the opposite of
what the creator intended, or they target the user instead of someone
else. The interesting point to keep in mind here is that these items
aren’t always bad to have. (DMG, p. 272)
A DM could allow a cursed wand of Mindless Rage that forces its user to attack the target. (Downside: you would have to use a standard action to activate the wand and have enough ranks in Use Magic Devise).
Maybe one could also think of a cursed Ring of Spell Storing, which directs its stored spells onto its user instead of the intended target. If the spell Mindless Rage would be cast into such a ring, it could be used by a barbarian (but would also take a standard action).
There are no clear rules on how cursed items are created in the DMG. You need some „malicious, chaotic or insane spellcaster“ and usually have to add a Bestow Curse spell in the process of creating.
Conclusion: I don't believe these ideas are of much help, as they make your barbarian partly lose control over his actions and put him at high risks. But since you asked for anything that gets around the limitation of one rage per encounter, I gave it a go. :)