Show me the rules quote where it says I have to hold a staff in two hands to benefit from it as an implement.
There isn't one, that's why every time this topic gets brought up it goes on for multiple pages.

Show me the rules quote where it says I have to hold a staff in two hands to benefit from it as an implement.
Duct tape the orb to the top of the staff.
Wow, that is a great question. PHB p. 236 says of Holy Symbols: "Unlike other implements, you need only to wear a holy symbol for its property of power to function." That suggests you need to do more with the orb + staff than just have them in contact with you. You can wield a weapon in each hand and choose which one you want to attack with if one of the weapons is labeled "Off Hand" (PHB p. 217), and I can't believe a wand wouldn't be considered as light and balanced as the weapons listed as off-hand-able. Orbs are heavy though (PHB 238), staffs are two-handed as weapons, so I wouldn't buy an argument that either should be considered off-hand-able. Nothing clearly stated in the text, I might buy an argument that you ought to be able to wield a wand in your off hand and choose between that and an implement in your primary hand. I can't find anything suggesting a staff or orb should be treated that way.
I can't find anything suggesting a staff or orb should be treated that way.
Show me the rules quote where it says I have to hold a staff in two hands to benefit from it as an implement.
Hmm, ya know, I'm not sure. I thought I saw it earlier but now all I see is that you have to wield it, page 157, and it is a staff, and a two-handed weapon and it says it is too heavy or unbalanced to be used without two hands.
You could argue that this is for the melee aspect of a staff as opposed to the implement aspect, however, so you're right...I gotta keep looking. Or maybe sleep.
Tellerve