I would definitely try using one pistol at a time at first rather than trying to lug two around. Keeping one pistol full of paint and gas is hassle enough.
As for your cartridges and tubes, the cheapest way is to pick up a nylon 12-gauge shotshell belt from your local Wal-Mart. Should cost around $8 and will hold more tubes and cartridges than most specialized stock-class vests and packs that are out there.
One word of advice though- don't store the belt full of tubes and 12-gram cartridges. Store it empty.
If you do, the elastic will last longer.
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