To the second poster, in FERS, SSA is considered part of your retirement package. Therefore, if you apply and qualify for both FERS disability payments and SSDI, then there is an offset. The offset is to your OPM disability retirement benefit, not to your SSDI.
For most employees, a FERS disability retirement is paid at a much higher percentage than a regular retirement. That's the basic the reason for the offset.
In your particular case, however, your FERS disability retirement would be recomputed to a regular retirement at age 62. The years on disability are added to your years of service, but, in your case you are only looking at one, maybe two additional years of service, so that a 1% to 2% increase in your regular retirement annuity at 62. Moreover, if you qualify for a regular retirement under MRA plus 20, then you would not be eligible for a disability retirement under FERS anyway.
I would run the numbers. It may not pay to file for a disabilty retirement if you would only recieve the higher benefit for one year or so and maybe a slightly enhanced benefit at age 62. If you cannot retire under an MRA plus 20, you probably could still retire under MRA plus 10. There is no FERS supplement in an MRA plus 10, but you would not be eligible for the supplement for that long of a period anyway at your age.
With SSDI, you are not subject to the early retirement penalty that occurs when you take them at age 62. While SSDI converts to regular SS retirement at age 66, the gross benefit amount does not change. That's likely to be much more beneficial to you in the long run than a 1% increase in your FERS annuity from age 62 forward.