As Martin said, it won't do much for you. A few years ago, having an external blog on the likes of blogspot was common practice, but it turns out that it was fairly easy for Google to figure out what was going on, seeing as they own blogspot.

You'd be better spending the time on other areas (local citations etc).

Who owns the sites redirecting? It's pretty rare to have someone redirecting to a site they don't own or have any business relationship etc with...

If it's the same person, and the redirects are spammy/not sending useful traffic, you can just turn the redirects off.

If the agency is working on your behalf, then it depends on the agreed contract. Ultimately, you might decide that it's worth switching to an agency which isn't as protective about the accounts.
Personally as someone who runs Adwords and Youtube accounts for various clients, I tend to group them within the account for my agency simply because it means we have a bigger overall spend, so we might get a slightly better response from Google etc, and it also means that we never have access issues when we're checking them daily (and the client generally doesn't have the time or desire to monitor as closely). But I also give them top level account access as a matter of course, even if they never use it - optimising ad accounts isn't a massive secret - it's mainly long, dull and tedious optimisation!

And if a client prefers, I'm always fine with letting them maintain the account as long as I can get the access I need - the only difference really is that it doesn't then count towards the totals for Google Partner status etc, but that's not a massive issue...

Having had agencies hand over accounts in the past, I've seen good and bad examples, and I certainly wouldn't want to work with an agency that wasn't willing to let me have a quick look at negative keyword lists etc every so often, espencially if you're spending a reasonable amount...

The way to think about it is as an opportunity, rather than a cost.

Instead of having two competing brands for the same customer, you now have the opportunity to target two segments of potential customers much more effectively.

Plenty of examples exist of multiple brands under the same umbrella targetting different customer groups. For example in drinks, look at how many brands Coca Cola own - and why they'd buy brands like Innocent to get to a different market group....

In finance, you might want to target one brand at younger consumers who will then grow into the other brand.

Or one for those on a lower budget, for example.

And that will naturally lead to different and unique content. I supply a huge amount of content for businesses, and my structure, language and tone has to be different for each one to appeal to the customers they want.