When debugging in intellij, I can tell it to display a given int variable with its decimal value or its hex value.
Is it possible for the debugger to display BOTH at the same time?
No, it's not possible.
When debugging, you can right click on an integer variable and choose View As > and either Hex or Primitive, not both.
This setting only applies to the single variable and doesn't seem to persist through different debugging sessions.
I had a look at the possibility of using a Data Type Renderer (right click a variable and go in to Customize Data Views... to see the dialog).
A Data Type Renderer only applies to complex data types (not primitives). So if for example your int that you wanted to see 38 and 0x26 was always part of a complex object (e.g. Person.age) then perhaps a data type renderer could help.
Alternatively you could add a String watch expression that constructs a String value for both the decimal and hex value of your int.
Sorry that there is not a way to do exactly what you want but I hope these 2 other suggestions are useful to consider.