SAP Netweaver Portal

I am going to give a typical SAP professional answer : IT DEPENDS !

well ...all jokes apart looking at your siutaion, since you already have a add-in installation or what some people call it dual stack system to run ESS in your company here is my advice :

you will face some disadvantages especially when restarting the dual stack system as you have to wait for java to completely start and only then wait for the ABAP stack to restart as well....I mean it becomes very difficult to restart ABAP quickly. a dual stack system is ideal for solution manager and in my recommendataion and experience DO NOT use this on the portal environment. consider a scenario when in future your CIO decides to have the user management by the desktop folks who only use active directory and do not wish to give them SAP access since a dual stack system uses SU01 to lock/unlock users on the portal it becomes quite a nightmare to move to the active directory as your UME....with the central system with AS JAVA only you can have the benefit of choosing ABAP or JAVA initially while installing the system and then doing the right choice of picking JAVA and then moving the UME to the active directory. it also makes things simple when you have SSO with SPnego when a user logs on to the portal and he single signed on and if his password changes on the network he only needs to call helpdesk and change his password which makes life easy.....versus having a dual stack system where ur user management is tied to the ABAP system and you will be left wondering how to resolve the user lock and unlock issues !!!

since you already have this scenario and you plan to do a BI Portal integration use a central system with usage types AS JAVA , EP, EPC. I understand that this is more expensive since you will now have 2 portal systems, but this again DEPENDS if you have a FPN with the producer and consumer portal synchronization is best suited using a central system and NOT the dual stack system.
again there is downside to this as well.......you have to ensure that the service pack for BI JAVA and EP are the same and if there is some patching to be done there will be severe downtime in both the environments and that is something most companies frown upon......but this is something to keep in mind...the key in integrating BI and Portal is the memory and you have to make sure sizing is done correctly along with the memory usage on the BI reports as well as sufficient load balancing in terms of defining how many server processes are necessary to run it efficiently ....there are certain notes that i can guide you...723909 - Java VM settings for J2EE 6.40/7.0 and Note 1044330 - Java parameterization for BI systems

so in the end it depends on the number of users using ESS and number of users using BI reports.......but I would still go for a brand new server (Central system with Java only and AD as your UME) and avoid the dual stack system....