| Avoron |
The most convenient way is to just have more arms. Alchemists, mutation warriors, and mutagenic maulers can take the vestigial arm discover, while kasathas get extra arms for free.
The other methods require a bit of juggling - juggler bard is traditional. Feral gnasher barbarians get a handy ability to pick up objects as a free action, allowing you to free up hands by dropping crossbows and picking them back up. Finally, if your crossbow's aren't enchanted you don't even need to reload, you could just drop each crossbow after firing and Quick Draw a new one.