Given that the differences between your strains probably involve rearrangements and copy number variations, this approach seems inadvisable if your goal is a highly contiguous genome. You will make the dBG into more of a hairball if such differences exist.
Illumina is quite cheap these days, I would vastly prefer simply sequencing more deeply if your issue in inadequate coverage. Online tools exist for figuring out how to get there.
Unfortunately, you will swiftly plateau in terms of contiguity even for quite simple genomes using Illumina. So you can expect quickly diminishing returns.
Knowing more stats on your assembly would help evaluate this. For example, if you are working with effectively 10X coverage with N50 ~ 500bp, I might be willing to look the other way while you co-assemble such a synthetic metagenome, because you will at least be able to assemble gene length contigs, which are unlikely to vary too dramatically between your strains. However, if the issue is that your assembly has an N50 ~ 10Kbp and you want it to be higher, then you should consider using other approaches to improve your assemblies, such as long reads. In a pinch, you could do something like optical maps or Hi-C, though I believe that is not preferred these days given much cheaper/better long reads. (Full disclosure: I used to work at a Hi-C company and still hold stock.)
The field of metagenomics is based on the assumption that your suggested approach is ok to get a basic idea of e.g. gene complement, and if you are clever you can even deconvolute such mixtures into genomes, under the assumption that different genomes are at most distantly related. But that doesn't mean that it's a good idea to do it in silico by combining isolates.
