What is the specific model of the router provided from your internet provider?
Do you get a few static ip's with your internet package or just dynamic?
I am using a CISCO ASA series to do a very similar configuration.
[We divide bandwidth based on branch offices]
Your HP device is a switch it would be better to setup two routers behind the providers router since they are separate companies.
If you are on a budget you can get ASUS routers that offer bandwidth management, QOS and WMM. Demo of the user interface is online at http://demoui.asus.com/index.asp I use these for remote workers. They allow to prioritize business applications over the kids gaming when an employee is working from home.[You could also do the reverse]
Not knowing the router or if you have multiple static IP's I can provide some possible options for you.
I would use two routers besides the one provided from your internet provider.
Have the one provided from the your bandwidth provider disable NAT; allow passthrough; otherwise this will cause you to traverse multiple NAT's. This is not the default configuration but can be requested from your provider. I know comcast for business, optimum and FIOS support this option.
Use the 192.168.200.xx range on one company and 172.16.10.x range on the second. Do not use 192.168.1.x; 192.168.0.x; 192.168.100.x as they are default IP ranges for various devices.
Setup static routing between the providers router and the routers for each business. Having separate routers for each company would be prefered for both security and liability issues. Using this type of config you can have DHCP on each network w/o conflict. Verify the wired computers from each company are connected to the correct router.
The ASUS devices work well for prioritization but not necessarily capping. This would give priority to the business that needs the majority of the bandwidth. It would also allow the second company to utilize the bandwidth if it is not in use.
You can use hard caps if you want but there is no real reason to do so.
Verify the provider's router is capable of QOS, packet prioritization if it does not use a third ASUS or NETGEAR in the same class. Once you have specific models lookup their capabilities relating to bandwidth management, QOS, packet prioritization, as well as WMM.
These routers also allow for guest networks and work over 2.4Ghz and 5Ghz wireless frequencies.
Disable the wireless on the main router if available to force users through the correct connection.
Check frequencies so that the two remaining routers are not using the same channels. Auto scan feature is also on these routers. If another wireless device pops up in the vicinity the router can change its channel. Turn off wireless networks if they will never be used. Many of the mid range consumer devices have 4 channels transmitting at once at a minimum between the 2.4Ghz network; the 5Ghz network and the guest networks on each.
Are you using wireless phones - if so use above channel 6 for the 2.4Ghz range
Alternate config on a budget:
You can physically configure this network using 1 good ASUS/NETGEAR router. i just would not for security and liability reasons. With that said if you choose to use this you would set one company on the primary network and one on the DMZ network. The time it would take to configure would exceed the cost of having one router for each company.
I am using ASUS RT AC68-P [about 179.00]in one location and a higher models at other locations. They are also full VPN servers not just pass through devices. Support USB devices for NAS storage.etc We frequently using multiple cloud services as well as telephony services for remote workers and they work well. At one location we are supporting eight employees for VOIP and data. We split the networks to isolate VOIP to its own network and corporate data on another network. There are two wires run for each office to completely isolate VOIP; 1 for data and 1 for Voice network.