Send your coins to yourself.
You can send all your coins to one address, putting them all in the same pot until the next stake (which may then divide them into two, depending on your wallet settings). It’s like taking several partly full cups and emptying them into one big staking cup.
If you have coin control switched on (recommended only for experienced users), you can choose which inputs to combine and the number of equal outputs. Multiple outputs all go to the same address but stake separately, just like coins coming from external sources.