From what the other Member said there is no advantage to having a separate Battery only for the Glow Plugs. However, in cold weather having a dual Battery set up would give you more capacity for Cranking the Starter and I also believe with 2 batteries the Voltage drop during cranking would be less. Higher voltage can mean higher Starter cranking speeds. But is there room for another Battery.
Another option is to get the highest capacity Battery that will fit in your car. However, I have read that some High Capacity Batteries need an Alternator that will put out at least 13.7 volts or they will not charge.
Besides using electric Block Heaters I have read of people bringing their battery inside of the house where it is warm.
JC Whitney also used to sell pad that went under the Battery that you plugged in to keep your battery warm. As the temp drops your battery performance drops also.

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84 300D, 82 Volvo 244Gl Diesel