Yes it is a good idea. It is very common to split meds containing T3. Also, fo info, you will get the full effect of T3 in serum thyroid hormone levels in about 4-5 days. The T4 portion will not get up to over 90 % of its final effect on serum levels for about 4 weeks.
Patients often find that their body needs some time to adjust when starting on thyroid meds. The body needs to heal after being hypo for some time.
You are correct 5 mcg of T3 a day. However, it's combined with T4. So splitting the T3 means splitting the T4 as well. Is that a good idea?
So your T3 dosage is now 5 mcg per day, right? If so, do you take it all at once in the morning? If so, a better approach is to split any T3 med and take half in the morning and the other half in early afternoon. Since T3 affects you so much faster than T4 med, that approach tends to even out the effect over the whole day.