Unless you had some custom super fancy frame, all water bottles were attached with straps until the end of the 70s. Almost all water bottle cages were TA. The straps to hold them on came with them, and were once a ubiquitous bike shop item. VO is the only place that still sells them. FWIW that's the historically correct way to do it. The standard move if adding a second cage was to put it on the seat tube. The frame pump then was moved to the top tube, or sometimes behind the seat tube if there was room. Top tube pump location requires a pip or umbrella clip, and a velcro strap for safety.
Anyhow, BITD, that's what I would do on a really hot day. Strap on a second bottle to the seat tube. Move the pump.
Moving to the modern day, most bottle cages won't even work with the old style straps. If working with a vintage bike, King sells those modded band clamps that should do the trick. I think the seat tube is still the best place for a second bottle. Handlebar cages are a viable option. Weight on the bars does slightly effect handling. If you've got a handlebar bag anyway, you could put a bottle in there. My modern classic bikes have dual cage braze ons. DT and ST. For camping, I'll take a couple Nalgene 32oz bottles and stuff 'em into my panniers. FWIW they fit into the pocket of Carradice Kendals. There's also those modern bladder things. Probably a good idea.
Which reminds me, when touring circa the early to mid 80s, standard configuration for me at least was: downtube tube bottle, seat tube bottle (w straps), and a bota bag for extra water. Yep, good old bota bags. Flexible hydration reservoir long before their time...