Harold Kestenholz
This would be a matter for the local code official. The manufacturer literature specifies for distances from combustibles; the appliance is not considered a combustible. the manufacturer also specifies clearances for service and venting. These are the ordinary specifications. Appliances sold as one product include these specifications. Independently sold appliances do not usually include directions for other appliances.
The remainder is prudence. If the new installation will make it difficult to test the relief valve, drain periodically, service the appliances and their gas controls or create a danger from surrounding machinery, then these must be corrected. The official may be concerned about replacement or removal in the future as well. The local officials make the decision.
