My environment:

Ubuntu : Ubuntu 24.04 Desktop
Windows : Windows 11

The key point is to obtain the MAC address, LTK, and IRK of a Bluetooth device paired on one system, and then write this information into the registry of the other system.

First, pair the Bluetooth device on Ubuntu, then pair it on Windows, so that both systems store the device’s information (we only need to modify the Address, LTK, and IRK later), and ensure Windows can use the device.

Windows Configuration

Download https://learn.microsoft.com/en-us/sysinternals/downloads/psexec?ref=xavier.wang

After downloading, open PowerShell as administrator, and in the program’s directory, open the registry with this command:

.\\PsExec.exe -s -i regedit

Navigate to the registry key:

HKEY_LOCAL_MACHINE\\SYSTEM\\CurrentControlSet\\services\\BTHPORT\\Parameters\\Keys\\<Computer's MAC address>/<Bluetooth device's MAC address>

Note down the LTK, IRK, and Bluetooth device address.

Ubuntu Configuration

On Ubuntu:

Note that the Bluetooth address is similar to the one found on Windows (because the device address changes after each reset, usually incremented by one):

cd /var/lib/bluetooth/<Host MAC address>/<Bluetooth device address>

View the info file:

[General]
Name=ERGO M575
Appearance=0x03c2
AddressType=static
SupportedTechnologies=LE;
Trusted=true
Blocked=false
Services=00001800-0000-1000-8000-00805f9b34fb;00001801-0000-1000-8000-00805f9b34fb;0000180a-0000-1000-8000-00805f9b34fb;0000180f-0000-1000-8000-00805f9b34fb;00001812-0000-1000-8000-00805f9b34fb;00010000-0000-1000-8000-011f2000046d;

[IdentityResolvingKey]
Key=8FC6396AB31C0E6AB68E37ED5BB7CC72

[SlaveLongTermKey]
Key=313498DE3D05749C6B20D393BEF36FB1
Authenticated=2
EncSize=16
EDiv=0
Rand=0

[ConnectionParameters]
MinInterval=6
MaxInterval=9
Latency=44
Timeout=216

[DeviceID]
Source=2
Vendor=1133
Product=45095
Version=9

[ServiceChanged]
CCC_LE=2

Be sure to modify the IdentityResolvingKey (IRK) and SlaveLongTermKey (STK).

After modification, rename <Bluetooth device address> to <Windows Bluetooth device address> and then restart the system.

References:

https://www.insanelymac.com/forum/topic/268837-dual-boot-bluetooth-pairing-solved/page/2/
https://www.insanelymac.com/forum/topic/318483-keep-bluetooth-devices-paired-once-between-macwindows/page/2/
https://bbs.pcbeta.com/viewthread-1034129-1-1.html
https://github.com/digitalbirdo/BT-LinkkeySync/tree/master
https://ubuntuforums.org/showthread.php?t=1479056