Probably the most practical place, you just need a short length of hose from machine to filter, to allow the filter to lie flat on the floor
Andrew, you won't notice any difference at all running 2 short lengths of 2" into a Y right near your machine. The manifold on the machine is a Y junction, you're just replicating the same thing a few feet further forward. Where you'll notice a slight bit of difference is if you run 50 or 100 foot of dual hose into a Y on a long hose run.
I did a school last year where I needed 200ft for several rooms; I ran 4" for the first 150' and really did notice a difference - ran it on mid speed rather than flat out and it cleaned beautifully with the RX. On reflection though afterwards, I thought to myself what did it really gain me? Added an extra 20 minutes to the job time plus faffing around loading/unloading my spare lengths of hose from the garage at home......and probably meant an unmeasurable reduction in drying time - after all, needlefelt school carpet in July with all the doors and windows open isn't really all that critical.
If I actually NEEDED 4 to the door on a job (due to air flow drop) the hose run would have to be so long I'd need about 12 lengths of vac hose. The 7 lengths I own are plenty! I ain't got the money to waste or the storage space in my garage for that!!
Only exception is one restaurant I do with a filthy greasy contract axminster that I need to obliterate to get clean, so I want to squeak every last cfm of airflow from my system to minimise shrinkage (always does in places) So I run 50ft of 4" to the Y and then 50ft of 2" to the RX-20, and machine on full power. Sucks like a 'you know what' with bills to pay!