Sleep needs change and develop as baby grows. It just so happens that your baby is getting to the stage where 2 nap days start. It might not be a straight swap to 2-nap days. I would take a month or two to transition so that by about 9 months 2-naps are established.
Both awake time and overall sleep matters. It's about balancing both and managing the changes as baby's sleep develops with age.
For example, I'd suggest around 15 hours sleep in 24 hours for the whole of the first 2 years. So a 6 month old having the same amount of sleep as a 12 month old and an 18 month old and a 24 month old. That does not mean a baby's sleep at 6m, 12m, 18m and 24m is identical.
Routine changes as baby develops, but it's awake time that changes, not sleep needs until much older. As baby grows, they can cope with longer awake, but this doesn't mean they need less sleep. Just fewer naps. For example:
4 month old:
45m sleep in every 2h between 7am-9am
Then about 9h out of 10h overnight sleep.
So about 16h sleep, 7 daytime naps.
6 months old:
9am-11am nap
1pm-3pm nap
5pm-6pm nap
About 10h (interupted) night sleep between 8pm-7am.
So about 15h sleep, 3 daytime naps
12 month old:
9am-11am nap
2pm-4pm nap
11h uninterputed night sleep between 8pm-7am.
So about 15h sleep, 2 daytime naps
18 month old:
9.30am-10.30am nap
1.30pm-4pm nap
11.5h uninterputed night sleep between 7.30pm-7am.
So about 15h sleep, 2 daytime naps
24 month old:
12pm-3pm nap
12h uninterputed night sleep between 7.30pm-7am.
So about 15h sleep, 1 daytime naps