Probably you are not backing up both iPhone 10 and 11 in the same iCloud storage. If that is not true then...
Please check you have enough iCloud Storage.
Please check iCloud available storage vis-a-vis the backup space required.
Alternative: Choose what to backup and what not to.
Settings > [Tap your Name] > iCloud > iCloud Storage > Backups > [Select your Device, if you have more than one] >
Here You have the next backup size
Choose data to backup. Disable the apps which may consume more space and are not important to you. Note this only backup data within App, not the app itself.
Do not forget to tap "Show all Apps" to see all the apps' data that is getting backed up.
Watch the GIF below to know the size of the next backup.