[Razer Phone 2 Guides, News, & Discussion] omega552003:
Correction I got it working, there's some key things that OP touched on but didn't elaborate. I'll try to make it a bit more clear for others that want to do this mod.

Requisites/environment:

QPST 2.7.477 - only PDC is used here, so any version with standalone PDC tool might do the job. But new versions required if you want to use other tools.

QXDM Professional 3.14.1144 - this one is optional. It's used to tweak some parameters, but dual SIM does work without the tweaks.

Not needed to just get it working​

Qualcomm USB drivers 2.1.2.0 from 2015/7/8 - older versions might work as well.

Windows 10, 1803

I used Windows 10 1909​

arter97's kernel and Magisk-patched stock kernel images readily at hand. The latter can be obtained by patching a stock kernel image in Magisk Manager with "Keep dm-verity/avb 2.0" UNTICKED. I had the latter installed on the phone.

OK this is the first BIG thing; You need to patch the STOCK kernel in Magisk with the 'Keep dm-verity/avb 2.0' unchecked!You can revert back after this is done.​

Make sure root shell can be obtained during boot with ALL kernel images. This is an important recovery approach if the phone bootloops.

You need to be running the stock kernel, arter97's will put you in a bad spot if you need to fix the reboot to recovery on login.​

1. make a backup of all partitions on the phone, leaving out system, vendor and userdata partitions of course. There are 88 partitions in my backup.

This is overkill in my opinion. Back up you data, make a back up to Google, grab the ROM from Razer's site, patch the boot.img with Magisk​

2. make a QCN backup of modem NV. This step could be optional as modemst1/2 has already been backed-up in previous step - I'm not sure about this, and have completed this step whatsoever.

Don't waste your time, this will get reconfigured by the carrier when you reconnect.​

3. make a backup of /vendor/etc/vintf/manifest.xml, and add slot2 instances to the following HALs:

Basically you're going through "manifest.xml" and duplicating every line that has "slot1" and changing it to "slot2". Also for every <instance>slot1</instance> there's a <fqname> with a slot1.​

android.hardware.radio

There are 2 instances in here​

vendor.qti.hardware.data.connection

vendor.qti.hardware.radio.am

I don't think this needed to be done​

vendor.qti.hardware.radio.qtiradio

There are 2 versions "1.0" and "2.1", one right after the other​

3.a Using the phone's shell, we need to replace "manifest.xml" back in to the /vendor partition. Don't try and get smart and think you can make a Magisk module to "inject" this. It doesn't work.

# mount -o rw,remount /vendor
# cp /sdcard/Download/manifest.xml /vendor/etc/vintf/

4.

adb shell su -c setprop ro.multi.rild true
adb shell su -c setprop persist.dsds.enabled true
adb shell su -c setprop persist.multisim.config dsds
adb shell su -c setprop persist.radio.multisim.config dsds

5. find any USB mode with rmnet in /vendor/etc/init/hw/init.msm.configfs.rc, and switch to it. I used "diag,serial_cdev,rmnet,dpl,adb", and that's

adb shell su -c setprop sys.usb.config diag,serial_cdev,rmnet,dpl,adb

6. Windows shows a lot of new USB devices. Force-install Qualcomm WWAN driver for the RmNet device. I chose "Network Adapters -> Qualcomm -> Qualcomm HS-USB WWAN Adapter 90B8", but anything named after "Qualcomm HS-USB WWAN Adapter" should do.

7. open PDC. There are 3 4 dual SIM (DSDS) hardware profiles available:

8. Reboot verify it works.

If you get a "Power Off" right after boot, its probably because your radio hardware profile isn't activated.

If you have 2 sim slots, but one doesn't have an IMEI, then your manifest.xml isn't done right.

I Selected the LA variant and so far so good, I have WiFi Calling and Dual Sims.

---------- Post added at 02:21 PM ---------- Previous post was at 01:46 PM ----------

To fit the second Sim in the tray, just shave the plastic on the side with out the notch so metal is at the edge it fits perfectly in the stock tray.