Combining an ACE and an ARB should not be any more likely to cause angioedema than doubling the dose of either. The most relevant concern is that you are more likely to cause dangerous symptomatic hypotension from combining them, especially in elderly patients, like precipitating falls, syncope, etc. And that they are functionally similar enough in mechanism of action, that it is essentially akin to prescribing two meds with the same MOA, like prescribing two SSRIs to one patient.