Yeah, two 30 mg ERs is the same as one 60 mg ER. The tricky part is how long they release and anything else you might take in that window. Say, they release over the course of 12 hours. Immediate release usually decreases in your system after 4-6 hours and if you've taken that med before, then you can probably tell. But ER will continue to release over its entire course and you have to treat anything else you might ingest with that in mind. That's what can make ERs a higher risk for overdose. At, say, 8 hours you might no longer be getting symptom relief from a 12-hour ER, but it's still in your system and taking anything else would add to the amount in your bloodstream and the load on your organs.
Does that make sense? An accidental double dose of an IR formulation is going to affect you very differently than a double dose of ER.