$6000 with tires (3 firestones and 1 continental)
The following parts needed to go from singles to duals on a 2388 and others

240851A1 Axle extensions x2
184456C2 Rim for 20.8x42.  x4
 
Platform extension containing:
124315A1
STEP
10'' Extension
1-2
  2
141588A1
HANDRAIL
12-3/4'', For 10'' Extension
4
  2A
195625A1
HANDRAIL
12-3/4'', Upper, For 10'' Extension
1
  3
141589A1
HANDRAIL
22-3/4'', For 20'' Extension
4
  3A
195627A1
HANDRAIL
22-3/4'', Upper, For 20'' Extension
1
  4
127719A2
SHIELD
For 10'' Extension
1
  5
127720A2
SHIELD
For 20'' Extension
1
  6
143819A1
SUPPORT
2-4
  7
143822A1
SUPPORT
1
  8
409-7616
BOLT
8
  9
231-5146
FLANGE NUT
8
  10
413-528
BOLT
4
  11
231-1445
LOCK NUT
4
  12
413-672
BOLT
8
  13
231-5146
FLANGE NUT
8
  14
477-82512
SERRATED SCREW
1
  15
231-5144
SERRATED NUT
1
  16
260-1448
SELF-TAP SCREW
4