18x10inch wud be Ok for one full grown axie. Remember axies can grow up to 30cm (12 inches), so as you have 2 in your current tank you may find that it gets a bit squashed for them in a couple of months. Our biggest adult axies are 28cm-30cm./and our 7 1/2mth olds 20-22cm long.
Some people use the 30cm rule (30cm taking into account an axie can grow to that size) or 45cm per axie (to give room for hidey/plants/decos). So, ideally, for 2 axies - 2 to 3ft tank (24in x 12inch/36in x 12inch); 3 axies - 3 - 4ft tank (36x12in - 48inx12in)
Also, if u do intend getting a third axie, it is best to quarantine it, separate container, for 30 days in case it passes on any nasties/illness or parasites to your current axies.