E-mail reputation

If you mean keep changing providers or multiple senders, then yes it can have an impact.
If starting to use a new provider, your IP needs to be warmed up over a period of time to not impact your deliveribility rate. Making sure you have your DKIM set up.

Please see email deliverability tips in various threads or here:
http://www.slideshare.net/jdavidhill/marketo-email-reputation-management

I am assuming you mean that the Brand is sending off of two ESPs (senders) at once. This is a fairly common practice for large brands to do as some ESPs offer different services that suit their needs. If each ESP is using a different from domain and IP address then your sending reputation would not be affected by actions between ESP #1 and ESP #2. However, links within the email messages can cause inbox placement issues if the landing page domains are affiliated with spam.