You can create a class derived from Print that forwards its output to either or both Serial and Serial1. The only method you need to implement for this to work is write(uint8_t):

class DualPrint : public Print
{
public:
    DualPrint() : use_Serial(false), use_Serial1(false) {}
    virtual size_t write(uint8_t c) {
        if (use_Serial) Serial.write(c);
        if (use_Serial1) Serial1.write(c);
        return 1;
    }
    bool use_Serial, use_Serial1;
} out;

You would use it like this:

out.use_Serial = true;
out.println("Printed to Serial only");
out.use_Serial1 = true;
out.println("Printed to both Serial and Serial1");
out.use_Serial = false;
out.println("Printed to Serial1 only");

Note that with this approach, unlike yours, printing number will format them as text only once, and the underlying Serial and Serial1 will only handle the resulting characters.

You can create a class derived from Print that forwards its output to either or both Serial and Serial1. The only method you need to implement for this to work is write(uint8_t):

class DualPrint : public Print
{
public:
    DualPrint() : use_Serial(false), use_Serial1(false) {}
    virtual size_t write(uint8_t c) {
        if (use_Serial) Serial.write(c);
        if (use_Serial1) Serial1.write(c);
        return 1;
    }
    bool use_Serial, use_Serial1;
} out;

You would use it like this:

out.use_Serial = true;
out.println("Printed to Serial only");
out.use_Serial1 = true;
out.println("Printed to both Serial and Serial1");
out.use_Serial = false;
out.println("Printed to Serial1 only");

Note that with this approach, unlike yours, printing number will format them as text only once, and the underlying Serial and Serial1 will only handle the resulting characters.

Edit: To answer the question in OP's comment, the construct

class ClassName
{
    ...definition...
} classInstace;

is a shotcut for

class ClassName
{
    ...definition...
};
ClassName classInstace;

A very similar construct exists in plain C:

struct struct_name {
    ...the struct fields...
} struct_instance;