It is going to depend on the gas demands of the appliances, and even if they could ever be used at the same time.

Gas lines are rarely installed any larger than required.
The most 'extra' capacity you usually get is the rounding up to the standard pipe size.

Thanks, Brickeyee. If I understand you correctly, we should probably have the gas FP starter capped off (or however it would be disconnected/removed) if we want to use that line for the grill. Is that right? So, when I get an estimate and the plumbing/gas contractor says we can just tap into the same line and still use the starter, it may be a good idea to move on to the next guy on my list.

I'd like to think that I could trust the plumber to do it right, but around here I've seen way too much that doesn't come close to code and the county inspectors don't notice or care. The folks who post in these forums have saved us a lot of grief (& money) over the years.

I think it's a good bet that the gas grill will never be used when it's cold enough to light a fire in the fireplace. And a gas starter is very short term intermitant use. rarely more than once a day for 10-15 min even during the coldest weather.
I wouldn't worry about leaving them both connected.
What I would worry about is if the line is big enough to run a good sized grill even with out the log lighter. A log lighter is pretty low demand so it's probably plumbed with 1/2" pipe. check the grill specs. and the gas pipe size. I've never seen gas pipes smaller than 1/2" but it could happen.

My dad had an N-gas grill in the backyard of the house I grew up in and had trouble with spider nests / webs between the shutoff valve and the burners. Pain in the butt to clean out. Not sure if new designs eliminate this possibility or not. (We grill year-round so we don't have to clean the broiler pan.) :-)

Thanks for the tip, Qwertyui. After I read your post, I located the installation instructions on Weber's site and printed the applicable pages to show the plumber so he will know what's required. As for cold weather grilling, dh does that a lot..."neither rain nor snow..." (kind of like mail carriers.) I did put my foot down at him holding a golf umbrella over the grill (& himself) in the middle of a thunderstorm; it would be nice if he lived to enjoy retirement.

Sofaspud, dh's propane grill hasn't attracted spiders so I'll hope the NG one won't either. I think the spiders here are too busy with the scorpions on the rock outcroppings to bother with much else.

Paid for "feasibility study" (a/k/a estimate) by BBB "A+" rated company. Existing line is 5/8", so they'd keep the starter connection. Cost approx. $1K. Deal breaker wasn't cost (although that was a bit of a shock) but their reluctance to pull a permit - insisted it wasn't needed, even though bldg. dept. emphatically says otherwise. Company would charge (unknown) extra $ if we demand permit as that would entail a return visit for the inspection. Fair enough, but why not include that in estimate? There were two other (safety) issues that gave me pause, but by then I'd already written off the idea.

Husband decided he will buy an extra propane tank, as he has a safe location in which to store it, and I'll start pricing the propane version of the grill he likes. At least we know it's possible should a future prospective buyer ever ask.

Thanks for your advice.