Just to add an alternate point of view, I absolutely would not drop the dummy. 8-9 months is about when babies develop the dexterity to find and put their own dummy and at that point dummies come into their own for a no-crying sleep trigger that isn't feeding.
I also would be shortening awake times in the afternoon and adding a third teatime powernap.
Not unusual for the transition from 2 to 3 naps to take time can remain until 9 ish months old. I cannot begin to imagine how demanding and exhausting it must be to keep a tired 7 month old going from 1pm to bedtime.
More sleep breeds better sleep is my mantra. Rather than limiting the number of sleeps to two (as you are) I would be limiting awake time to hours through the daytime and then 3 hours before bed. How long baby naps then dictates how many naps.
Some examples might be:
Four 45 min naps, late bedtime
6.30 wake
8.30-9.15 sleep
11.15-12.00 sleep
14.00-14.45 sleep
16.45-17.30 sleep
20.30 bed
Three 45 min naps, early bedtime
6.30 wake
8.30-9.15 sleep
11.15-12.00 sleep
14.00-14.45 sleep
18.00 bed
Four mini 30 minute naps
6.30 wake
8.30-9.00 sleep
11.00-11.30 sleep
13.30-14.00 sleep
16.00-16.30 sleep
19.30 bed
Three 1h naps
6.30 wake
8.30-9.30 sleep
11.30-12.30 sleep
14.30-15.30 sleep
18.30 bed
... or whatever. Here is a random length naps example
6.00 wake
8.00-9.20 sleep (80 min nap)
11.20-12.50 sleep (30 min nap)
14.50-15.30 sleep (40 min nap)
19.00 bed
Point is, rather than fitting a square peg in a round hold and expecting two longer naps, I advocate more sleep is better sleep and to always make sure baby is getting plenty of sleep. So if naps are short, keep them frequent.
Nap lengthen naturally from 5m-9m ish. Once extended, naps automatically becomes less frequent.