A tee for the drain is fine, with a check/one way valve on both ro drains. Without a reservoir tank for the fridge you will wait 10 minutes for a glass of water, and the line going to ice maker will be more prone to freeze because without the reservoir it will flow so slowly.
The other thing about the ro for fridge is when the unit first goes on tds is higher for first few minutes, until unit gets pressurized and it will only be on for a few minutes each time. This tds will settle in reservoir and build up over time. Imo if your source water isn’t that bad, or you run a whole house water filter, and a regular fridge filter gets you south of 50 ppm tds would go without ro for fridge.