Faster spoolup is a tricky word.
The smaller secondary turbo spools first then as exhaust flow really picks up, the larger primary turbo spools up. For maximum power, you want to open the wastegate early so that more of the higher temperature higher velocity exhaust gasses can spin the large primary turbo. Generally speaking, the harder you drive the primary, the more power you will make.
At high altitude, the thin air makes turbos lazy. Your best bet for "off-idle" smoke control is to tighten the wastegate. If you keep the wastegate closed, all of the exhaust energy will be used to fully spool up the smaller secondary turbo. The smaller turbo is where you get the bottom 1/3 of your power.
Think about it this way, if you open the wastegate when the top turbo is making 20psi boost, it will be delayed on building additional boost because you are bypassing half of it's exhaust flow. This in turn will give the larger bottom turbo more exhaust so it will spoolup faster but if tuned wrong, you will create a flat spot "dead spot" in the power band as the top turbo is half spooled and bottom turbo is trying to spool.
If keep the wastegate closed till the top turbo is making 45psi, it will be fully spooled and making enough air for good power and good smoke control. The downside is that it will take longer for the bottom turbo to spool up since the wastegate is opening so late.
Obviously there are trade offs and you just gotta play with it till it runs best for you. If the top turbo is huge like a 66mm silver bullet, it's going to spool slow at high altitude no matter how you setup the wastegate adjustment.
It would probably be worth your time to dismantle the top turbo and inspect the wastegate to ensure it is functional and not bend open or burned. A loose wastegate can cause all kinds of smoke and sluggish spoolup issues.