Two females can get along just fine.
Now that you are going to have a mixed gender pair, it is going to be critical that you do everything necessary to prevent breeding.
Make sure you don't put a nest box OR anything in the cage that can possibly be used as a nesting site. Limit daylight hours to 10 per day but if you see signs of the budgies trying to mate, you'll need to decrease the daylight hours to 8 per day.
When either of the budgies come into condition, limit the amount of protein in their food at that time.
I use an actual cage cover but you can easily use a cotton tablecloth or curtain.
You can find inexpensive ones in a size that will be suitable for your cage at Walmart.
You want to use a breathable fabric and I recommend a dark color to block out more light. (My cage covers are black)
Cover the top, three sides and 1/3 of the way down the front of the cage.
Be sure you have a nightlight on in the room to give the budgies a bit of light to help prevent night-frights.
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