A twin-set is sometimes used by divers, so it isn't that difficult, with some of the cylinders charged to 300 bar. The cylinder regs connect to a metal pipe, with the single line coming off it to the demand valve, or in your case, paintball reg.
To reduce strain on the cylinder regs & connecting metal pipe, a spacer is placed between the cylinders with metal bands around the cylinders to keep them together. You may need to manufacture these items yourself as I haven't seen this set-up using small cylinders eg 1.5 litre bottles. Have a look at some dive shop websites for a general idea of what's needed.