I've had similar experience (COX here but also with a business HSI account) with both connections run to the same location - both businesses run out of the same office and I'm the admin for both.
In my case, one of the connections is set up with a block of static addresses that are managed and routed internally with the provided modem acting only as a gateway device; and the other modem acting as a router and using NAT translation to a dynamic IP provided by COX. Initially I was seeing higher throughput on the dynamic
connection. I thought perhaps it had to do with the routers on my side adding latencies to the static connection - but after further investigation I determined it was a router on the COX side. I don't want to tell you how long I had to fight with their tech support before I escalated the issue to someone in their NOC that actually had the knowledge and authority to adjust the router tables appropriately - but after they did I now see the same bandwidth on both connections.
I'm not saying this will be the case for you necessarily - however, it could be something as simple as them setting the throttling incorrectly for your connection. All cable connections have a throughput limit that's imposed by the hardware (both the physical copper and the modem itself) as well as the limitation of the aggregate - because it's shared
bandwidth. However, these limitations are far, far higher than you are ever allowed to use - from a sales standpoint.
I have the exact same connection equipment at my home as I do at my office. However, at home I'm capped at 15Mb/3Mb and at work I have 50Mb/5Mb connections. Since DOCSIS 2.0 the per channel bandwidth is limited physically
to 38Mb/27Mb (DL/UL) but with DOCSIS 3.0 this limitation is expanded indefinitely by allowing the local muxing of multiple channels (i.e. it is very possible to have a single point of service with over 100Mb of bandwidth in either direction by bonding 4 channels of 38/27).
All of this is a long way of saying that the software running at the ISP is entirely responsible for almost every aspect of your connections 'limitations' - and it's usually managed by people that are barely able to turn on a PC without assistance (OK that's an exaggeration - but not by that much). So the first thing I would do is call support, and then keep calling until you get someone that can affect a change to your service level. If either of your connections are transmitting at less than your indicated service level - then they BOTH should be transmitting at the same level... because you are both on the same node. Anything else would indicate either a configuration issue or perhaps a premise wiring issue after the distribution point (in which case you would need to fix that).
EDIT - I just re-read your post (carefully this time)
On second thought calling your support will only kill your connection rates entirely. You're both getting way more bandwidth than you are paying for!!! In my case I was not getting anywhere close to the listed rates on the one connection (due to a painful amount of packet loss ultimately) so that was a real problem.
If I could even get 1Mb more in either direction on my home connection than I was paying for... I wouldn't care if my neighbor was getting 2Mb extra. Free bandwidth is exactly that! Congratulations!
In all likelihood, unless you were using the same machine on both connections - it could be a simple matter of your side's configurations or something like a server or workstation running an unattended update download at the same time.Edited by DiGiCiDAL - 11/13/12 at 6:37pm