how to phrase a conditional condition?

I want to query a self join that will only join on the first condition and only if it cant find a match it will join the second condition

I tried this syntax which doesnt work for me, because it will understandbly always join both conditions

SELECT a.id, a.id2, a.col1, b.id2 from test a
LEFT JOIN test b ON a.id = b.id AND 
(SUBSTRING(a.col1, 1, LEN(a.col1) - 2) = b.col1 OR b.col1 = '00')

What I need is like an XOR instead.

Example: table

desired result

Data

CREATE TABLE [test](
[id] [int] NOT NULL,
[id2] [int] NOT NULL,
[col1] [varchar](6) NOT NULL
)
INSERT INTO [test] VALUES
(4080,2147,'00'),
(4080,2148,'0001'),
(4080,2149,'0002'),
(4080,2150,'000201'),
(3820,2069,'00'),
(3820,2070,'000101')

You can do two separate left joins and then pick the "best match" as needed.

For example:

select a.*, 
  case when a.col1 = '00' then null else coalesce(b.id2, c.id2) end as bid2
from test a
left join test b on b.id = a.id 
  and SUBSTRING(a.col1, 1, LEN(a.col1) - 2) = b.col1
left join test c on c.id = a.id and c.col1 = '00'

Result:

 id    id2   col1    bid2 
 ----- ----- ------- ---- 
 4080  2147  00      null 
 4080  2148  0001    2147 
 4080  2149  0002    2147 
 4080  2150  000201  2149 
 3820  2069  00      null 
 3820  2070  000101  2069 

See running example at db<>fiddle.