When booking Marriott, where do you enter your FF program number?
 

I don't see any where that allows you to enter FFP number during the booking process at Marriott.com

I have Gold Elite with Marriott, but I want to earn emirates miles for my upcoming stay.

Go to my account, Profile

https://www.marriott.com/rewards/myAccount/profile.mi

Expand Earn Preferences, click on miles, then choose your program.

so if I want to earn miles from two different FFP from two different Marriott bookings, I have to change the FFP twice in my profile page?

Yes. It's not really set up for constantly switching.

You also won't earn your elite bonus if you opt to earn miles. So as a gold you're missing out on 25% bonus.

It is a given that you won't earn the bonus since you won't earn the base.

I am fine with that. I am a 550 emirates miles short to be able to use my remaining emirates miles. And my current Marriott account balance is 0 points, so I don't think I would miss out too much if I miss on two nights worth of Marriott points.

another question though.

Will food / baverage charge (and also parking charge) in my Marriott room bill be able to earn Emirates miles (2x miles per dollar spent)? Or only room rate before tax counts toward earning 2x Emirates miles?

what about the rate booked with "Marriott Rewards Member Exclusive Offer"? Would this be eligible room rate?

depends on the Marriott brand you're staying at as a few of them limit points/miles to room only charges.

I'll be staying at a Marriott and a renaissance..... if I spend on room service and parking, would these charges count as 2x miles per dollar spent?

room service billed to the room - yes

and those charges earning miles do not include miles for service/tax

parking - depends on whether parking is owned by individual property

BTW, all your current and previous questions are all answered here -
https://www.marriott.com/marriott-rewards/earn/miles.mi

I saw that... But it only says "qualifying charge", but does not define what a qualifying charge is, not even in t&c.... so I am not really sure what constitutes a "qualifying charge"...

your question regarding how to enter FFP # and what is a "qualifying charge" are very basic but your original post states you're Marriott Gold - are you sure since your questions sound more like you've never stayed at a Marriott or had any MR background.

Like most companies, MR has all sorts of qualifiers on just about any statement they make. Generally, anything that can be billed to your room will be covered. Yes, there are exceptions especially where the service provider is not directly tied to the property (i.e. parking valets, laundry services, and some restaurants/gift shops). There is no definitive list that can be issued program wide and some may not be obvious when on site. Your best approach is to ask when there and take names just in case what they tell you is wrong (way too often).