Yes, you'll need a standalone to properly run 550cc injectors. There are a few oldschool piggyback options that would 'work' but these days it's just not worth the effort to learn all the intricacies needed.

Personally I would not bother with anything smaller than 750cc or so since you're going to have to remap everything to work on a modern standalone anyway. It will be a substantial outlay of cash to properly wire, configure, and fully tune a good modern standalone.

The front turbo is the #1 turbo and the rear one near the firewall is #2. The #1 turbo runs exclusively in low RPM and the #2 comes on and they both run at higher RPM. There are a few ways in the old 'BPU secret sauce' methods of raising boost on both somewhat to smooth out the power curve but basically anything you do to raise total boost level will do the trick. Stu Hagen has a neat trick to increase pre-spool boost pressure on #1 to make more torque earlier. #1 Turbo EBV Boost Controller
I would be careful with any elevated boost levels on the VVTi engine or a JDM engine because of the ceramic turbos that are much more fragile at higher boost levels than the stock USDM twins that Stu Hagen was using at the time.

Here's a great thread with pictures that explains how it all works: Comparing the 2JZ-GTE and 13B-REW Sequential Turbos
Please do not post in that thread, it's 10 fucking years old.

Stu Hagen also developed a fantastic sequential twin turbo kit that upgrades almost everything in the sequential setup, and retains sequential operation while offering 700+ whp and 130+mph trap speeds. It's a fantastic thing, but since Speed For Sale closed up I'm not sure where you can source it. It does require a standalone and someone very familiar with the stock sequential system to get properly dialed in, though.

 

Everything Jess said and...

1.2 bar aka 17.5 psi is the most I'd push through the stock twins on the JDM cars. Keep in mind, the 2JZ-GTE VVTi runs a MAF and MAP sensor and the MAF is different to the Export (aka US/Euro) spec MAF.

The stock twins, standalone, larger injectors (I'd suggest ID 1300x minimum for future proofing), uprated fuel pump (OEM Denso), FMIC (always go larger than you need - future proof!!) and boost control on both the first / second turbo's will be a nice fun torquey car. Once the stock twins give up on you, as they did with me after a lot of abuse, it's time to go single which, if you go with larger injectors and standalone, you'll simply be looking at an exhaust manifold, turbo, wastegate and the piping involved.

Oh, and plug up the stock lines to the recirculated blow off valve, there's no need for a BOV and you'll notice the car picks up a bit of power (leaky recirc stock BOV) and runs smoother overall.