Here are some suggestions for research you could try while you are waiting for the death certificates to come in. Keep in mind that New York City keeps its own records, separate from New York State.
Review your prior research
You don't say how you know that Isaac and Allegra Honen were your second great-grandparents. Make a timeline of everything you know about them, noting how you know it.
Make a list of all your sources, arranged in the order they were created, and cross-reference this list with your timeline. Review all the sources you have on hand to see if there is information you may have missed. If you have sources where you don't know when they were created, put them at the end of the list (as if they were today's date).
Do your ancestors have any siblings? Add that information to a family timeline, and examine the entire family's information as a group.
If you have information that has passed down through the family, write that down and note as much as you can remember about who told you and when. If you are not in the oldest generation of your family, interview the older generation; if you have siblings, interview them. Write down what you learn. You can cite these interviews as "conversation with [name] on [date]", "email from [name] on [date]" and so on.
It is smart to review starting with the records created at death, or after death (like probate records) because those can give the fullest picture of a person's life.
FamilySearch Wiki Resources
The article Substitute Records For United States Death Information lists several different types of information that might hold information about someone's death and burial. Since you have exact deaths of dates already, newspaper research may be a good place to start.
The article New York City, New York Genealogy gives an overview of other record types for New York City that you might want to look for.
The article New York City, New York, Online Genealogy Records list records available online, both free and by subscription.
If you have difficulty finding other records from the time before their deaths, you may be able to get help from questions already on the site, or by posting a new question.