How to extend the water changing time in an aquarium for two juvenile red eared slider turtles?

No. It’s like having to scoop a litter box. It is just part of the maintenance of pets.

Since turtles can carry Salmonella bacteria clean conditions are important.

I have a pair of pet geese and I have to dump and scrub their swimming pools every other day rain or shine. Our pets deserve to live in clean and healthy surroundings. Don’t neglect your little turtles, take pride in providing a great habitat for them. :-)

Get a bigger tank, obviously. Throw a second filter in too.

You really need a bigger tank, for one of the smallest turtles you should start by having about a 30 – 50 gallon tank yours is about 45 going on your measurements and is therefore too small for your two now they have grown up.
This is what they say at that website :

For the smallest turtles, start with at least a 30–50 gallon (113–189 liter) glass aquarium (see Water before you rush out and buy that 30 gallon aquarium you saw on sale!) . If you are not interested in actually being able to watch your turtle swimming around under water, you can use a suitably large opaque plastic container such as a large plastic storage box bottom, concrete mixing bin or deep kitty litter pan. You can use clean aquarium rock and gravel to build a slope up from the wet end (the pool) to the dry end (the land). You can silicone together pieces of Plexiglas to make a moveable platform onto which your turtle can crawl onto to rest. Floating or anchored cork rafts or logs are another alternative. Rough rocks must not be used as they can scratch turtle shells which allows bacterial and fungal infections to get started and penetrate into the turtle’s body.
Note: one of the biggest mistakes aquatic turtle keepers make is not providing a body of water that deep, long and wide enough for their turtle. The minimum size required for a 4” turtle will not work for a 6” or 8” (15 or 20 cm) aquatic turtle, and certainly not for a full grown one. Since turtles will grow relatively quickly when they are cared for properly, you should start off with an enclosure size big enough for your turtle to comfortably grow into for at least 1–2 years. That will give you some time to think out, plan, and build the turtle’s next, much larger, enclosure.

Think two turtles are better than one? Assuming they are compatible, it can be nice for your turtles to have one another for company. But two turtles require an even larger enclosure than a single turtle. So, unless you are prepared to keep and service giant enclosures for turtles who can easily reach the size of dinner plates, rethink getting two…or even one.

In other words: your size tank is the right size for one small turtle. Not two larger ones.

Bigger tank with more filtration. I actually own two RES turtles myself, and they’re currently in a 30g tank with a DIY filter I made. The water is crystal clear and I haven’t changed it in months.

I think your filter may be too weak, get something with a higher GPH, and a bigger tank.

RES turtles are hardy, you don’t need to have super clean water all the time. They can survive in pond-like conditions. You can let the water go a little murky, but just keep an eye out for health signs.