I personally asked this a few months back; I'll link to the thread if I can find it. The short answer is no... electronic certificates cannot be combined with paper vouchers. You'll find that you cannot do a "book and hold" on united.com if you apply an e-certificate.
However, there is one anecdote of an FTer ticketing in-person at an airport, and the agent there apparently said she could enter an e-certificate code... in that case, you should be able to combine an e-certificate with another voucher. However, since the PMO/DOLLARUA is a Type A voucher, you would be combining two discount instruments, which is usually not allowed; usually only one discount instrument is allowed, although you could combine that with multiple cash-value instruments (e.g. Type B vouchers).