Dear S.P Krishna,
I think, the switch option is the only option
. Since on a rare occaision a person may switch on both amps accidently, and one of the amps may get FRIED
(I think one of our members "Pradski" has mentioned such incident in this forum, you can use search to get more details)
Your case sounds like deja vu to me. I am also proud owner of 5105... faced the same SQ dillemma... added Wharfdale 9.2 as fronts (dramatic improvement!!!)... sometime later realised that just spending 10k on Norge 1000 can make another dramatic improvement... so got Norge 1000 also . Now I think I have the best HT+Stereo combo at overall cost of 60K approx.
About speaker choice:
I haven't heard Usher 520, but read rave reviews about it. If budget permits (26k approx) you should go for them. If you are bass lover, you will find them anemic (reviews say that). However since you have Onkyo 5105's active subwoofer, you wouldn't miss so much (set subwoofer crossover at 100 Hz). After sometime you can add Norge 1000 for another 10k and get another jump in SQ.
Substitute Usher 520 with Wharfe 9.2 at 14k approx. 9.2's bass extension is unbelievable at this price! (Wharfe 10.1 is reported to have some boomy bass issues and placement problems since its bass reflex ports are at rear... so wouldn't recommend that) + buy Norge 1000 rightaway with savings resulting from not buying Usher 520!
Audition Usher 520 and Wharfe 9.2 BOTH. If you like laid back sound, You may actually find the 9.2 "sufficiently" detailed, and overall a better performer. Whereas people who like more details in treble would like Usher 520.
In Hi-fi, the journey is sometimes more enjoyable than destination. So here's wishing Happy Journey !!! to you.