I think you were incorrectly advised and/or are mixing modifier usage.
Modifier 25 is appended to E/M procedures to indicate a significant, separately identifiable E/M.
Modifier 50 is appended to procedures (which allow the use of 50) to indicate a bilateral procedure. The exact same on both sides for paired organs or structures.
If the documentation of the E/M supports a modifier 25 with the procedure performed on the same date, you would append a 25.
If the documentation also supports bilateral major joint injections (such as the knees), you would append a modifier 50 to 20610 or 20611 on one line with one unit double the fee (as suggested above). I have seen cases where a RT shoulder and LT knee were done at the same time and the payer actually still wanted 20610-50 with the two different dx codes. Seems weird. I would normally do two lines for two different joints.
There are some random payers (rare) who may want bilateral same joint type injections billed on two lines one unit each RT/LT.
Modifier 51 is pretty obsolete at this point. Most major payers and CMS do not want it because their internal claim systems already rank the codes correctly by RVU/multiple procedure reductions. Also, it makes no sense to append a 51 to a CPT which has the same RVU or fee because it won't matter when the claim is processed if they are the same. I guess you could still use it if you are ranking surgical procedures and want to make sure the first listed is the 100% line and the others are 50%, but again, most carriers don't want the 51.
Some references, you can check your specific MAC too.
www.novitas-solutions.com
"Note: Medicare doesn’t recommend reporting Modifier 51 on your claim; our processing system will append the modifier to the correct procedure code as appropriate."
In your example, depending on the payer guideline,
and provided the documentation supports it, it would probably look like this:
9920X-25
20610-50 (w/ US 20611-50)
J_____ (drug) w/ correct units for dose.