if you have either a belt or belt-loops on your hiking pants/shorts, try using some bungee to afix the Platys. if you can do this, then you've added only a miniscule amount of wt to your kit - viz. the two small loops of bungee (or four if you want added security - 2 loops for each Platy).
if you aren't already carrying a lot of wt. in your pants/shorts, there may not be an issue with the ~4lbs of water attached as stated above. the issue being, of course, your pants/shorts falling down! might be a problem, don't know. give it a try, unless you already know that it is an unworkable solution.
or, a 1/2" wide webbing belt with side release buckle (all necessary parts available from REI) could easily be fashioned. you could easily afix bungee loops to it & the 2 Platys to the bungee. the webbing belt gives you other options for carrying also, e.g. for climbing or squeezing through some rocks you might want to relocated the belt so as not to damage the Platys.
my guess is that you would want, if possible, the Platys to be more horizontally oriented about the waist vs. vertically oriented.
just some thoughts.