The way you combine the /24 subnets is to change the subnet mask to a /22. (ie. 255.255.252.0) This makes it all one contiguous address space. Just change the subnet mask to 255.255.252.0 and ...
I have an office, they are using the subnet 200.200.200.0/24 (254 ip addresses). They need more IP addresses, I have the 200.200.201.0/24 subnet available. If I want to give them both subnets for ...